Diagnosis of chorionicity: The role of ultrasound

array:21 ["pii" => "S2173412713000693" "issn" => "21734127" "doi" => "10.1016/j.diapre.2013.09.004" "estado" => "S300" "fechaPublicacion" => "2014-07-01" "aid" => "106" "copyright" => "Asociación Española de Diagnóstico Prenatal" "copyrightAnyo" => "2013" "documento" => "article" "subdocumento" => "fla" "cita" => "Diagn Prenat. 2014;25:58-64" "abierto" => array:3 [ "ES" => true "ES2" => true "LATM" => true ] "gratuito" => true "lecturas" => array:2 [ "total" => 21302 "formatos" => array:3 [ "EPUB" => 139 "HTML" => 20014 "PDF" => 1149 ] ] "itemAnterior" => array:17 [ "pii" => "S2173412713000681" "issn" => "21734127" "doi" => "10.1016/j.diapre.2013.09.003" "estado" => "S300" "fechaPublicacion" => "2014-07-01" "aid" => "105" "copyright" => "Asociación Española de Diagnóstico Prenatal" "documento" => "article" "subdocumento" => "fla" "cita" => "Diagn Prenat. 2014;25:53-7" "abierto" => array:3 [ "ES" => true "ES2" => true "LATM" => true ] "gratuito" => true "lecturas" => array:2 [ "total" => 7963 "formatos" => array:3 [ "EPUB" => 119 "HTML" => 6793 "PDF" => 1051 ] ] "en" => array:12 [ "idiomaDefecto" => true "cabecera" => "Original Article" "titulo" => "Monozygotic twins: Ten reasons to be different" "tienePdf" => "en" "tieneTextoCompleto" => "en" "tieneResumen" => array:2 [ 0 => "en" 1 => "es" ] "paginas" => array:1 [ 0 => array:2 [ "paginaInicial" => "53" "paginaFinal" => "57" ] ] "titulosAlternativos" => array:1 [ "es" => array:1 [ "titulo" => "Gemelos monocigóticos: 10 razones para ser diferentes" ] ] "contieneResumen" => array:2 [ "en" => true "es" => true ] "contieneTextoCompleto" => array:1 [ "en" => true ] "contienePdf" => array:1 [ "en" => true ] "autores" => array:1 [ 0 => array:2 [ "autoresLista" => "Alexandra Matias, Sara Silva, Yolanda Martins, Isaac Blickstein" "autores" => array:4 [ 0 => array:2 [ "nombre" => "Alexandra" "apellidos" => "Matias" ] 1 => array:2 [ "nombre" => "Sara" "apellidos" => "Silva" ] 2 => array:2 [ "nombre" => "Yolanda" "apellidos" => "Martins" ] 3 => array:2 [ "nombre" => "Isaac" "apellidos" => "Blickstein" ] ] ] ] ] "idiomaDefecto" => "en" "EPUB" => "https://multimedia.elsevier.es/PublicationsMultimediaV1/item/epub/S2173412713000681?idApp=UINPBA00004N" "url" => "/21734127/0000002500000002/v1_201411190048/S2173412713000681/v1_201411190048/en/main.assets" ] "en" => array:19 [ "idiomaDefecto" => true "cabecera" => "Original" "titulo" => "Diagnosis of chorionicity: The role of ultrasound" "tieneTextoCompleto" => true "paginas" => array:1 [ 0 => array:2 [ "paginaInicial" => "58" "paginaFinal" => "64" ] ] "autores" => array:1 [ 0 => array:4 [ "autoresLista" => "Vincenzo D’Addario, Cristina Rossi" "autores" => array:2 [ 0 => array:4 [ "nombre" => "Vincenzo" "apellidos" => "D’Addario" "email" => array:1 [ 0 => "vincenzo.daddario@uniba.it" ] "referencia" => array:1 [ 0 => array:2 [ "etiqueta" => "*" "identificador" => "cor0005" ] ] ] 1 => array:2 [ "nombre" => "Cristina" "apellidos" => "Rossi" ] ] "afiliaciones" => array:1 [ 0 => array:2 [ "entidad" => "Dept. Obstetrics and Gynecology, University of Bari, Italy" "identificador" => "aff0005" ] ] "correspondencia" => array:1 [ 0 => array:3 [ "identificador" => "cor0005" "etiqueta" => "⁎" "correspondencia" => "Corresponding author." ] ] ] ] "titulosAlternativos" => array:1 [ "es" => array:1 [ "titulo" => "Diagnóstico de corionicidad: papel de la ecografía" ] ] "resumenGrafico" => array:2 [ "original" => 0 "multimedia" => array:7 [ "identificador" => "fig0035" "etiqueta" => "Fig. 7" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr7.jpeg" "Alto" => 491 "Ancho" => 650 "Tamanyo" => 56555 ] ] "descripcion" => array:1 [ "en" => "Dichorionic twin pregnancy in the second trimester: two widely separated placental masses (P) can be seen, one on the anterior and one on the posterior wall of the uterus." ] ] ] "textoCompleto" => "IntroductionTwin pregnancies account for 1–2.5% of all pregnancies<a class="elsevierStyleCrossRef" href="#bib0005">1</a>; the prevalence of twins showed an increase of approximately 80% since the beginning of the 1970s, mainly due to the increase of the maternal age at conception over this period and to the widespread diffusion of the assisted reproduction technology.Twin pregnancies are at significant risk for adverse outcome with perinatal mortality and morbidity rate three to six times that of singletons. The main determinant of fetal outcome in twin pregnancy is placental chorionicity: monochorionic twins have a three to five higher risk of perinatal loss and handicap in comparison to dichorionic.<a class="elsevierStyleCrossRefs" href="#bib0010">2,3</a> This is mainly due to complications specifically associated with monochorionicity such as twin to twin transfusion syndrome (TTTS), twin reversed arterial perfusion syndrome (TRAP), selective intrauterine growth restriction (IUGR), which are consequences of the vascular anastomoses between the two twins sharing the single placenta.<a class="elsevierStyleCrossRef" href="#bib0020">4</a>The determination of chorionicity in a twin pregnancy is important to plan an intensive monitoring of the monochorionic pregnancies with the aim of improving their outcome, to perform accurately prenatal invasive procedures and to early diagnose and manage complications such as TTTS and selective IUGR.Zygosity and chorionicityZygosity refers to the type of conception: monozygotic twins result from the mitotic division of a zygote originating from a single ovum fertilized by one sperm; dizygotic twins are the result of a double conception originating from two ova fertilized by two sperms. Monozygotic twins account for 30% of all twin pregnancies. The origin of monozygotic twinning is still unclear and different hypotheses have been proposed, the most exiting one suggesting that monozygotic and dizygotic twinning events arise from the same embryogenic mechanism.<a class="elsevierStyleCrossRefs" href="#bib0025">5–7</a>Chorionicity refers to the type of placentation and does not reflect zygosity. Whereas dizygotic twins are always dichorionic, monozygotic twins may be monochorionic or dichorionic depending on when the zygote divides. This is the hypothesis of the Corner's theory<a class="elsevierStyleCrossRef" href="#bib0040">8</a> which has never been demonstrated in humans,<a class="elsevierStyleCrossRef" href="#bib0030">6</a> but is useful for understanding the development of different types of chorionicity in monozygotic twins. According to this theory, if the split of the zygote occurs during the first three days following fertilization, dichorionic diamniotic twins develop: this occurs in 29% of the cases. When the split occurs 4–7 days after fertilization a monochorionic diamniotic twin pregnancy develops: this occurs in 70% of the cases. In 1% of the cases the split occurs later (between 8 and 12 days) resulting in monochorionic monoamniotic twins. When a partial splitting of the zygote occurs between 13 and 16 days following fertilization conjoined twins develop; this is an extremely rare condition occurring in up to 200,000 births.Ultrasound diagnosis of chorionicityAlthough dizygotic twins always have a dichorionic placenta, the appearance of the placentas would depend on the sites of the blastocysts implantation. In case of distant implantations in the uterine cavity two separate placentas can be demonstrated; in case of close implantation they will appear fused in a single placental mass, similar to that of the monozygotic monochorionic twins. However, due to the higher risk of complications, monochorionic twins need an increased surveillance with strict follow-up and for this reason they must be differentiated from the dizygotic dichorionic twins with fused placentae as early as possible.First trimester The ideal time to determine chorionicity is the first trimester. As early as 4–5 weeks postmenstrual weeks, by using transvaginal sonography, it is possible to recognize the chorionic sacs inside the uterine cavity within the tick deciduas. They appear as sonolucent round structures with a brightly echogenic rim. By simply counting the chorionic sacs it is possible to establish whether the pregnancy will be monochorionic, dichorionic, trichorionic and so on (<a class="elsevierStyleCrossRef" href="#fig0005">Fig. 1</a>).<elsevierMultimedia ident="fig0005"></elsevierMultimedia>The visualization of a single chorionic sac at 4–5 weeks, however, does not exclude the possibility of a monochorionic twin pregnancy. By the sixth postmenstrual week the yolk sac and the embryo can be visualized inside the chorionic sac and the definitive diagnosis of a single pregnancy or monochorionic twin pregnancy can be done. If a single chorionic sac is visualized containing two yolk sacs and two embryonic poles, then a monochorionic twin pregnancy can be diagnosed (<a class="elsevierStyleCrossRef" href="#fig0010">Fig. 2</a>).<elsevierMultimedia ident="fig0010"></elsevierMultimedia>Determination of amnioticity is difficult until 8 weeks of gestation. Before that time the amnion is still too close to the embryo and is identifiable in only 50% of the cases.<a class="elsevierStyleCrossRef" href="#bib0045">9</a> The amnions appear as thin echogenic membranes surrounding the two embryos inside the celomatic cavity (<a class="elsevierStyleCrossRef" href="#fig0015">Fig. 3</a>).<elsevierMultimedia ident="fig0015"></elsevierMultimedia>Lack of visualization of the amniotic membranes after 8 weeks allows the diagnosis of monochorionic monoamniotic twins. In this case the number of yolk sac seen may be one or two depending on the timing of cell division.<a class="elsevierStyleCrossRef" href="#bib0050">10</a>After 10 weeks of gestation chorionicity is determined by the evaluation of the number of the placental masses and, in case of a single placental mass, by the characteristics of the membrane dividing the amniotic sacs. In dichorionic twins with fused placentas, amnios and chorion reflect away from the placental surface, creating a potential space into which villi can grow. Sonographically this space appears as a triangular-shaped extension of placental tissue into the base on the inter-twin membrane. This sign was first described in 1981 as the “lambda” sign<a class="elsevierStyleCrossRef" href="#bib0055">11</a> and is used interchangeably with the “twin peak” sign described in 1992<a class="elsevierStyleCrossRef" href="#bib0060">12</a> (<a class="elsevierStyleCrossRef" href="#fig0020">Fig. 4</a>). In monochorionic pregnancy there is a single layer of continuous chorion limiting the villous growth; the intertwin membrane takes off perpendicularly to the placental surface producing the sonographic sign known as “T” sign (<a class="elsevierStyleCrossRef" href="#fig0025">Fig. 5</a>).<elsevierMultimedia ident="fig0020"></elsevierMultimedia><elsevierMultimedia ident="fig0025"></elsevierMultimedia>Several studies have confirmed the accuracy of “lambda” and “T” signs in predicting chorionicity in the first trimester, comparing the sonographic diagnosis with the placental hystology or the discordant sex at birth.<a class="elsevierStyleCrossRefs" href="#bib0065">13–17</a> The sensitivity reported ranges from 89,8% to 100%; the specificity ranges from 97.4% to 99.8% (<a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>). The gestational age at the time of the sonographic examination varied in the different studies. Stenhouse et al.<a class="elsevierStyleCrossRef" href="#bib0065">13</a> and Menon et al.<a class="elsevierStyleCrossRef" href="#bib0075">15</a>report a sensitivity of 100% but they examined the patients at 7–14 and 5–14 weeks respectively, thus including in these good results also the patients examined in the earliest stages of pregnancy (below 10 weeks) when the sonographic diagnosis relies on the number of the chorionic sacs rather then the T/lambda sign. The study by Lee et al.<a class="elsevierStyleCrossRef" href="#bib0080">16</a>reported four sex discordant twin pairs with monochorionic placentas. They used this finding to contest the belief that monochorionicity is synonymous with monozygosity. However they did not perform genetic tests to confirm zygosity. Carroll et al.<a class="elsevierStyleCrossRef" href="#bib0175">14</a> compared the accuracy of T/lambda sign and membrane thickness and found that membrane thickness improved sensitivity by 1.3%, although its assessment is time consuming and highly inter- and intraobserver dependent. The largest series is by Dias et al.<a class="elsevierStyleCrossRef" href="#bib0085">17</a>: they examined 613 twin pregnancies with transabdominal ultrasound at the time of the routine first trimester screening (11–14 weeks): sensitivity and specificity were 100% and 99.8% respectively. The only error occurred in a pregnancy complicated by a small hematoma running along the site of the membrane insertion.<elsevierMultimedia ident="tbl0005"></elsevierMultimedia>Another technique to assess chorionicity in the first trimester is the evaluation of the number of layers of the intertwin membrane, considering that the dividing membrane is made by only two layers of amnios in monochorionic and four layers (two amnios and two chorions) in dichorionic twins. This technique, mainly used in the second and third trimester, has also been applied in the first trimester using transvaginal approach and depicting the amnios and chorion at the membrane to placenta insertion.<a class="elsevierStyleCrossRef" href="#bib0090">18</a>In conclusion the accuracy of ultrasound in the assessment of chorionicity in the first trimester is high and is mainly based on the evaluation of lambda and T-sign.Second and third trimester. Determination of chorionicity in the second and third trimester is more problematic and the diagnostic accuracy is lower than in the first trimester. The diagnosis relies on the evaluation of fetal gender, number of the placental masses and characteristics of the intertwin membrane.<a class="elsevierStyleCrossRef" href="#bib0095">19</a>Fetal gender The identification of discordant fetal gender indicates dichorionic twinning (<a class="elsevierStyleCrossRef" href="#fig0030">Fig. 6</a>). The positive predictive value of discordant gender (when correctly identified) is 100%. However, considering that around 50% of concordant sex twins are dichorionic, the definition of chorionicity needs further sonographic signs.<elsevierMultimedia ident="fig0030"></elsevierMultimedia>Number of the placental masses The location of the placental masses has a limited value in the assessment of chorionicity. Separate placental sites are an indicator of dichorionic twins (<a class="elsevierStyleCrossRef" href="#fig0035">Fig. 7</a>). However widely separated placentas are found in only one-third of all twin gestations. Furthermore two separated placental masses may be seen in the rare cases of a monochorionic placenta which is bilobed or has a succenturiate lobe. For this reason the sensitivity of two placental sites is 32% and the PPV is 97.7%.<a class="elsevierStyleCrossRef" href="#bib0100">20</a> A single placental mass may be indication of both monochorionic and dichorionic twins with fused placentas (<a class="elsevierStyleCrossRef" href="#fig0040">Fig. 8</a>).<elsevierMultimedia ident="fig0035"></elsevierMultimedia><elsevierMultimedia ident="fig0040"></elsevierMultimedia>Given these limitations, identifying chorionicity must rely on other sonographic parameters particularly when a single placental mass is seen.Take-off of the intertwin membrane The ultrasonic visualization of the intertwin membrane origin (take-off) from the placental surface in the second and third trimester has a limited diagnostic value as compared to the first trimester. The membrane take-off to recognize lambda and T-sign may be obscured by the fetus, particularly in the third trimester and when the placenta is posterior. Furthermore, it must be taken into account that as the pregnancy progresses the lambda sign may disappear as the consequence of the progressive regression of the chorion frondosum to form chorion leave at the base of the intertwin membrane. The lambda has been shown to disappear by the 20th week of gestation in about 7% of dichorionic twins with fused placentas.<a class="elsevierStyleCrossRef" href="#bib0105">21</a> This supports the conclusion that while the presence of the lambda sign indicates dichorionicity, its absence does not always rule it out.<a class="elsevierStyleCrossRef" href="#bib0110">22</a>Thickness of the inter-twin membrane The inter-twin membrane thickness is greater in dichorionic twins because it is made of four layers (two chorions and two amnios) as compared to monochorionic twins, whose membrane is made by two layers (two amnios) and consequently is thinner and hairlike.The membrane thickness can be measured with high resolution ultrasound equipments. The membrane should be imaged perpendicularly to the ultrasonic beam after magnification (<a class="elsevierStyleCrossRef" href="#fig0045">Fig. 9</a>). The most reliable site for measurement with the most reproducibility is close to the placenta (<3cm).<a class="elsevierStyleCrossRef" href="#bib0115">23</a> The cut-off value firstly suggested to differentiate dichorionic from monochorionic twins is 2mm. Using that value Winn et al.<a class="elsevierStyleCrossRef" href="#bib0120">24</a> report a diagnostic accuracy of 82% for monochorionicity and 95% for dichorionicity. Bracero and Bryne<a class="elsevierStyleCrossRef" href="#bib0125">25</a> report a sensitivity of 75.7%, a specificity of 85.7%, a positive predictive value of 96.6%, and a negative predictive value of 40.0%, for determining dichorionicity. A lower cut-off of 1,5mm has been suggested for measurements made in the first trimester.<a class="elsevierStyleCrossRef" href="#bib0175">14</a>The sensitivity of membrane thickness for determining chorionicity declines with increasing gestation with a sensitivity of only 52% for dichorionicity in the third trimester.<a class="elsevierStyleCrossRef" href="#bib0130">26</a><elsevierMultimedia ident="fig0045"></elsevierMultimedia>In order to improve the reproducibility of the membrane thickness measurement and to be sure that the membrane is perpendicular to the ultrasonic beam, three dimensional multiplanar sonographic evaluation has been suggested with a very slight increase of sensitivity.<a class="elsevierStyleCrossRef" href="#bib0135">27</a>Due to these limitations, to the technical difficulties and to the operator dependency, the measurement of the inter-twin membrane thickness is of limited value in determining chorionicity in the second and third trimesters.Number of the layers of the inter-twin membrane The inter-twin membrane is made by four layers (amnios-chorion-chorion-amnios) in dichorionic and only two layers (amnios-amnios) in monochorionic twins. Using high resolution equipment and magnified images with the membrane perpendicular to the ultrasound beam it is possible to recognize the layers; counting more than two layers allows to diagnose dichorionicity<a class="elsevierStyleCrossRefs" href="#bib0095">19–29</a> (<a class="elsevierStyleCrossRef" href="#fig0045">Fig. 9</a>). It is useful to visualize the membrane close to its placental insertion in order to minimize the risk of error due to the folding back of a monochorionic membrane on itself thus appearing to have four layers.By counting the number of layers in the inter-twin membrane a 100% predictive accuracy for dichorionic and 94,4% for monochorionic have been reported.<a class="elsevierStyleCrossRef" href="#bib0145">29</a> Another study reported a PPV of 100% for dichorionicity.<a class="elsevierStyleCrossRef" href="#bib0150">30</a> The technique is more accurate in the second rather than the third trimester due to the tinning of the membrane with progressing gestational age.<a class="elsevierStyleCrossRef" href="#bib0155">31</a>Counting of the layers in the inter-twin membrane is not commonly used in the clinical practice, because it is time consuming, technically difficult and operator dependent.Doppler evaluation of the placenta A further technique suggested to recognize monochorionic twins in the third trimester is the detection of arterio-arterial anastomoses using Doppler evaluation of the placenta. Using this technique Fichera et al.<a class="elsevierStyleCrossRef" href="#bib0160">32</a> report a sensitivity of 75% and a specificity of 100% with detection rate increasing with gestational age.Also this technique, however, is time consuming, operator dependent and requires highly sophisticated equipments and its use in determining chorionicity in the clinical practice is very limited.ConclusionsThe prenatal determination of chorionicity is the first step for an accurate managing of twin gestation. Knowledge of chorionicity helps in risk assessment, genetic counseling, invasive procedure and management of TTTS and selective IUGR, death of one twin and discordant fetal anomaly. It is best done in the first trimester, when the diagnostic accuracy approaches 100%. The most reliable sonographic signs are the lambda and T-sign and the number of the placental masses evaluated before 14 weeks of gestation.Despite the widespread diffusion of ultrasound as a basic screening test, there is still limited knowledge about the correct diagnosis of chorionicity. A recent study showed that only 43% of multifetal pregnancies carried an accurate diagnosis of chorionicity before tertiary care center evaluation.<a class="elsevierStyleCrossRef" href="#bib0165">33</a> For this reason if there is uncertainty in the diagnosis of chorionicity the patient should be referred to a specialist.The importance of a correct diagnosis of chorionicity in twin gestation has been recently stressed by the report of the National Collaborating Center for Women's and Children's Health on Multiple Pregnancy,<a class="elsevierStyleCrossRef" href="#bib0170">34</a> whose recommendations are:<ul class="elsevierStyleList" id="lis0005"><li class="elsevierStyleListItem" id="lsti0005">•Determine chorionicity at the time of detecting twin pregnancy by ultrasound using the number of placental masses, the lambda and T-sign and membrane thickness.</li><li class="elsevierStyleListItem" id="lsti0010">•Assign nomenclature to fetuses (for example: upper and lower, left and right) and document this clearly in the woman's notes.</li><li class="elsevierStyleListItem" id="lsti0015">•If a woman with a twin pregnancy presents after 14 weeks determine chorionicity at the earliest opportunity by using all the possible signs (number of placental masses, lambda and T-sign, membrane thickness, discordant sex).</li><li class="elsevierStyleListItem" id="lsti0020">•If it is not possible to determine chorionicity by ultrasound at the time of detecting the twin pregnancy, seek a second opinion from a senior ultrasonographer or offer the woman referral to a healthcare professional who is competent in determining chorionicity as soon as possible.</li><li class="elsevierStyleListItem" id="lsti0025">•If it is difficult to determine chorionicity even after referral manage the pregnancy as monochorionic until proved otherwise.</li><li class="elsevierStyleListItem" id="lsti0030">•Provide regular training so that ultrasonographers can identify the lambda and T-sign accurately and confidently. Training should cover ultrasound scan measurements needed for women who book after 14 weeks and should emphasize that the risks associated with twin pregnancies are determined by chorionicity and not zygosity.</li><li class="elsevierStyleListItem" id="lsti0035">•Conduct regular clinical audits to evaluate the accuracy of determining chorionicity.</li></ul>Conflict of interestThe authors declare no conflict of interest." "textoCompletoSecciones" => array:1 [ "secciones" => array:10 [ 0 => array:2 [ "identificador" => "xres383468" "titulo" => "Abstract" ] 1 => array:2 [ "identificador" => "xpalclavsec362440" "titulo" => "Keywords" ] 2 => array:2 [ "identificador" => "xres383469" "titulo" => "Resumen" ] 3 => array:2 [ "identificador" => "xpalclavsec362439" "titulo" => "Palabras clave" ] 4 => array:2 [ "identificador" => "sec0005" "titulo" => "Introduction" ] 5 => array:2 [ "identificador" => "sec0010" "titulo" => "Zygosity and chorionicity" ] 6 => array:2 [ "identificador" => "sec0015" "titulo" => "Ultrasound diagnosis of chorionicity" ] 7 => array:2 [ "identificador" => "sec0020" "titulo" => "Conclusions" ] 8 => array:2 [ "identificador" => "sec0025" "titulo" => "Conflict of interest" ] 9 => array:1 [ "titulo" => "References" ] ] ] "pdfFichero" => "main.pdf" "tienePdf" => true "fechaRecibido" => "2013-08-28" "fechaAceptado" => "2013-09-12" "PalabrasClave" => array:2 [ "en" => array:1 [ 0 => array:4 [ "clase" => "keyword" "titulo" => "Keywords" "identificador" => "xpalclavsec362440" "palabras" => array:3 [ 0 => "Twins" 1 => "Chorionicity" 2 => "Ultrasound" ] ] ] "es" => array:1 [ 0 => array:4 [ "clase" => "keyword" "titulo" => "Palabras clave" "identificador" => "xpalclavsec362439" "palabras" => array:3 [ 0 => "Gemelos" 1 => "Corionicidad" 2 => "Ecografía" ] ] ] ] "tieneResumen" => true "resumen" => array:2 [ "en" => array:2 [ "titulo" => "Abstract" "resumen" => "Chorionicity is the main determinant of the perinatal outcome in twin pregnancies: perinatal mortality and morbidity are significantly higher in monochorionic versus dichorionic twins. This is mainly due to complications associated specifically with monochorionicity, such as twin to twin transfusion syndrome (TTTS), selective fetal growth restriction (FGR) and twin reverse arterial perfusion syndrome (TRAP), consequences of the presence of inter-twin vascular anastomoses in the common placenta. For this reason the diagnosis of chorionicity in twins is of clinical importance in order to plan an increased surveillance in monochorionic gestations and to recognize the appearance of complications in their early stages.Different sonographic signs may be used to evaluate chorionicity: number of placental masses, sex of the fetuses, characteristics of the intertwine membrane. The last one is surely the most useful and valuable tool: the take-off of the membrane from the placental surface shows the typical “lambda” appearance in dichorionic pregnancy and the typical “T” appearance in the monochorionic ones. In this article the sonographic features that help in the accurate depiction of chorionicity are reviewed." ] "es" => array:2 [ "titulo" => "Resumen" "resumen" => "La corionicidad es el principal determinante del desenlace perinatal en los embarazos gemelares: la morbimortalidad perinatal es significativamente superior en los gemelos monocoriónicos que en los bicoriónicos. Esto se debe principalmente a las complicaciones asociadas con la corionicidad, a saber, el síndrome de transfusión fetal-fetal (STFF), la restricción del crecimiento fetal selectivo (CIR) y la perfusión arterial reversa (secuencia TRAP), que son debidas a anastomosis vasculares intergemelares en la placenta común. Por esta razón, el diagnóstico de corionicidad en gemelos es de importancia clínica, ya que así se podrá planificar una mayor vigilancia en gestaciones monocoriónicas y detectar la aparición de complicaciones en las primeras etapas.Distintos signos ecográficos sirven para evaluar la corionicidad: número de masas placentarias, sexo de los fetos, características de la membrana intergemelar. Esto último es, sin duda, la herramienta más útil y valiosa: la separación de la membrana de la superficie placentaria muestra una típica forma «lambda» en el embarazo bicoriónico y un típico signo en «T» en el monocoriónico. En este artículo se revisan las características ecográficas que ayudan a la descripción exacta de la corionicidad." ] ] "multimedia" => array:10 [ 0 => array:7 [ "identificador" => "fig0005" "etiqueta" => "Fig. 1" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr1.jpeg" "Alto" => 396 "Ancho" => 500 "Tamanyo" => 53318 ] ] "descripcion" => array:1 [ "en" => "Dichorionic twin pregnancy at 5 weeks 3 days. Two round sonolucent sacs with a brightly echogenic rim are clearly visible in the thick decidua." ] ] 1 => array:7 [ "identificador" => "fig0010" "etiqueta" => "Fig. 2" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr2.jpeg" "Alto" => 355 "Ancho" => 500 "Tamanyo" => 45248 ] ] "descripcion" => array:1 [ "en" => "At 6 postmenstrual weeks a single chorionic sac is seen containing two yolk sacs: the diagnosis of monochorionic twin pregnancy can be done; it is not possible yet to diagnose amnioticity." ] ] 2 => array:7 [ "identificador" => "fig0015" "etiqueta" => "Fig. 3" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr3.jpeg" "Alto" => 484 "Ancho" => 650 "Tamanyo" => 54037 ] ] "descripcion" => array:1 [ "en" => "Monochorionic diamniotic twin pregnancy at 8 postmenstrual weeks. The two amnions appear as thin echogenic membranes (arrows) close to the two embryos, inside the celomatic cavity." ] ] 3 => array:7 [ "identificador" => "fig0020" "etiqueta" => "Fig. 4" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr4.jpeg" "Alto" => 446 "Ancho" => 650 "Tamanyo" => 59204 ] ] "descripcion" => array:1 [ "en" => "Twin peak sign (arrow) in a dichorionic twin pregnancy in the first trimester." ] ] 4 => array:7 [ "identificador" => "fig0025" "etiqueta" => "Fig. 5" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr5.jpeg" "Alto" => 511 "Ancho" => 650 "Tamanyo" => 47879 ] ] "descripcion" => array:1 [ "en" => "T-sign (arrow) in a monochorionic twin pregnancy in the first trimester." ] ] 5 => array:7 [ "identificador" => "fig0030" "etiqueta" => "Fig. 6" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr6.jpeg" "Alto" => 493 "Ancho" => 650 "Tamanyo" => 55460 ] ] "descripcion" => array:1 [ "en" => "Dichorionic twin pregnancy in the second trimester. Arrows point to the fetal genitalia (female on the left and male on the right)." ] ] 6 => array:7 [ "identificador" => "fig0035" "etiqueta" => "Fig. 7" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr7.jpeg" "Alto" => 491 "Ancho" => 650 "Tamanyo" => 56555 ] ] "descripcion" => array:1 [ "en" => "Dichorionic twin pregnancy in the second trimester: two widely separated placental masses (P) can be seen, one on the anterior and one on the posterior wall of the uterus." ] ] 7 => array:7 [ "identificador" => "fig0040" "etiqueta" => "Fig. 8" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr8.jpeg" "Alto" => 837 "Ancho" => 1083 "Tamanyo" => 99401 ] ] "descripcion" => array:1 [ "en" => "Twin pregnancy in the second trimester: a single placental mass (A) is seen on the anterior wall of the uterus. The chorionicity cannot be determined on the only base of the placental masses." ] ] 8 => array:7 [ "identificador" => "fig0045" "etiqueta" => "Fig. 9" "tipo" => "MULTIMEDIAFIGURA" "mostrarFloat" => true "mostrarDisplay" => false "figura" => array:1 [ 0 => array:4 [ "imagen" => "gr9.jpeg" "Alto" => 1365 "Ancho" => 1083 "Tamanyo" => 60425 ] ] "descripcion" => array:1 [ "en" => "Evaluation of the inter-twin membrane in the second trimester. The membrane is imaged perpendicularly to the ultrasonic beam and magnified. (A) monochorionic twins: two layers may be seen and the thickness is below 2mm (B) dichorionic twins: more than two layers may be seen and the thickness is above 2mm." ] ] 9 => array:7 [ "identificador" => "tbl0005" "etiqueta" => "Table 1" "tipo" => "MULTIMEDIATABLA" "mostrarFloat" => true "mostrarDisplay" => false "tabla" => array:1 [ "tablatextoimagen" => array:1 [ 0 => array:2 [ "tabla" => array:1 [ 0 => """ <table border="0" frame="\n \t\t\t\t\tvoid\n \t\t\t\t" class=""><thead title="thead"><tr title="table-row"><td class="td" title="\n \t\t\t\t\ttable-head\n \t\t\t\t " align="left" valign="\n \t\t\t\t\ttop\n \t\t\t\t" style="border-bottom: 2px solid black">Author \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-head\n \t\t\t\t " align="center" valign="\n \t\t\t\t\ttop\n \t\t\t\t" style="border-bottom: 2px solid black">Pregnancies (n) \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-head\n \t\t\t\t " align="center" valign="\n \t\t\t\t\ttop\n \t\t\t\t" style="border-bottom: 2px solid black">GA (weeks) \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-head\n \t\t\t\t " align="center" valign="\n \t\t\t\t\ttop\n \t\t\t\t" style="border-bottom: 2px solid black">Sensitivity (%) \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-head\n \t\t\t\t " align="center" valign="\n \t\t\t\t\ttop\n \t\t\t\t" style="border-bottom: 2px solid black">Specificity (%) \t\t\t\t\t\t\n \t\t\t\t</td></tr></thead><tbody title="tbody"><tr title="table-row"><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="left" valign="\n \t\t\t\t\ttop\n \t\t\t\t">Stenhouse (2002) \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">96 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">7–14 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">100 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">98.7 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="left" valign="\n \t\t\t\t\ttop\n \t\t\t\t">Carrol (2002) \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">150 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">10–14 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">98 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">97.4 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="left" valign="\n \t\t\t\t\ttop\n \t\t\t\t">Menon (2005) \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">344 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">5–14 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">100 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">97.9 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="left" valign="\n \t\t\t\t\ttop\n \t\t\t\t">Lee (2006) \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">247 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t"><14 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">89,8 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">99.5 \t\t\t\t\t\t\n \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="left" valign="\n \t\t\t\t\ttop\n \t\t\t\t">Dias (2011) \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">613 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">11–14 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">100 \t\t\t\t\t\t\n \t\t\t\t</td><td class="td" title="\n \t\t\t\t\ttable-entry\n \t\t\t\t " align="char" valign="\n \t\t\t\t\ttop\n \t\t\t\t">99.8 \t\t\t\t\t\t\n \t\t\t\t</td></tr></tbody></table> """ ] "imagenFichero" => array:1 [ 0 => "xTab585853.png" ] ] ] ] "descripcion" => array:1 [ "en" => "Sensitivity and specificity of lambda and T signs for determining chorionicity in the first trimester of pregnancy." ] ] ] "bibliografia" => array:2 [ "titulo" => "References" "seccion" => array:1 [ 0 => array:2 [ "identificador" => "bibs0005" "bibliografiaReferencia" => array:34 [ 0 => array:3 [ "identificador" => "bib0005" "etiqueta" => "1" "referencia" => array:1 [ 0 => array:2 [ "contribucion" => array:1 [ 0 => array:2 [ "titulo" => "Births: final data for 2008" "autores" => array:1 [ 0 => array:2 [ "etal" => false "autores" => array:6 [ 0 => "J.A. 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Diagnóstico de corionicidad: papel de la ecografía

Vincenzo D’Addario

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vincenzo.daddario@uniba.it

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, Cristina Rossi

Dept. Obstetrics and Gynecology, University of Bari, Italy

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          "en" => "<p id="spar0055" class="elsevierStyleSimplePara elsevierViewall">Dichorionic twin pregnancy in the second trimester&#58; two widely separated placental masses &#40;P&#41; can be seen&#44; one on the anterior and one on the posterior wall of the uterus&#46;</p>"
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    "textoCompleto" => "<span class="elsevierStyleSections"><span id="sec0005" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0025">Introduction</span><p id="par0005" class="elsevierStylePara elsevierViewall">Twin pregnancies account for 1&#8211;2&#46;5&#37; of all pregnancies<a class="elsevierStyleCrossRef" href="#bib0005"><span class="elsevierStyleSup">1</span></a>&#59; the prevalence of twins showed an increase of approximately 80&#37; since the beginning of the 1970s&#44; mainly due to the increase of the maternal age at conception over this period and to the widespread diffusion of the assisted reproduction technology&#46;</p><p id="par0010" class="elsevierStylePara elsevierViewall">Twin pregnancies are at significant risk for adverse outcome with perinatal mortality and morbidity rate three to six times that of singletons&#46; The main determinant of fetal outcome in twin pregnancy is placental chorionicity&#58; monochorionic twins have a three to five higher risk of perinatal loss and handicap in comparison to dichorionic&#46;<a class="elsevierStyleCrossRefs" href="#bib0010"><span class="elsevierStyleSup">2&#44;3</span></a> This is mainly due to complications specifically associated with monochorionicity such as twin to twin transfusion syndrome &#40;TTTS&#41;&#44; twin reversed arterial perfusion syndrome &#40;TRAP&#41;&#44; selective intrauterine growth restriction &#40;IUGR&#41;&#44; which are consequences of the vascular anastomoses between the two twins sharing the single placenta&#46;<a class="elsevierStyleCrossRef" href="#bib0020"><span class="elsevierStyleSup">4</span></a></p><p id="par0015" class="elsevierStylePara elsevierViewall">The determination of chorionicity in a twin pregnancy is important to plan an intensive monitoring of the monochorionic pregnancies with the aim of improving their outcome&#44; to perform accurately prenatal invasive procedures and to early diagnose and manage complications such as TTTS and selective IUGR&#46;</p></span><span id="sec0010" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0030">Zygosity and chorionicity</span><p id="par0020" class="elsevierStylePara elsevierViewall">Zygosity refers to the type of conception&#58; monozygotic twins result from the mitotic division of a zygote originating from a single ovum fertilized by one sperm&#59; dizygotic twins are the result of a double conception originating from two ova fertilized by two sperms&#46; Monozygotic twins account for 30&#37; of all twin pregnancies&#46; The origin of monozygotic twinning is still unclear and different hypotheses have been proposed&#44; the most exiting one suggesting that monozygotic and dizygotic twinning events arise from the same embryogenic mechanism&#46;<a class="elsevierStyleCrossRefs" href="#bib0025"><span class="elsevierStyleSup">5&#8211;7</span></a></p><p id="par0025" class="elsevierStylePara elsevierViewall">Chorionicity refers to the type of placentation and does not reflect zygosity&#46; Whereas dizygotic twins are always dichorionic&#44; monozygotic twins may be monochorionic or dichorionic depending on when the zygote divides&#46; This is the hypothesis of the Corner&#39;s theory<a class="elsevierStyleCrossRef" href="#bib0040"><span class="elsevierStyleSup">8</span></a> which has never been demonstrated in humans&#44;<a class="elsevierStyleCrossRef" href="#bib0030"><span class="elsevierStyleSup">6</span></a> but is useful for understanding the development of different types of chorionicity in monozygotic twins&#46; According to this theory&#44; if the split of the zygote occurs during the first three days following fertilization&#44; dichorionic diamniotic twins develop&#58; this occurs in 29&#37; of the cases&#46; When the split occurs 4&#8211;7 days after fertilization a monochorionic diamniotic twin pregnancy develops&#58; this occurs in 70&#37; of the cases&#46; In 1&#37; of the cases the split occurs later &#40;between 8 and 12 days&#41; resulting in monochorionic monoamniotic twins&#46; When a partial splitting of the zygote occurs between 13 and 16 days following fertilization conjoined twins develop&#59; this is an extremely rare condition occurring in up to 200&#44;000 births&#46;</p></span><span id="sec0015" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0035">Ultrasound diagnosis of chorionicity</span><p id="par0030" class="elsevierStylePara elsevierViewall">Although dizygotic twins always have a dichorionic placenta&#44; the appearance of the placentas would depend on the sites of the blastocysts implantation&#46; In case of distant implantations in the uterine cavity two separate placentas can be demonstrated&#59; in case of close implantation they will appear fused in a single placental mass&#44; similar to that of the monozygotic monochorionic twins&#46; However&#44; due to the higher risk of complications&#44; monochorionic twins need an increased surveillance with strict follow-up and for this reason they must be differentiated from the dizygotic dichorionic twins with fused placentae as early as possible&#46;</p><p id="par0035" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleItalic"><span class="elsevierStyleBold">First trimester</span></span> The ideal time to determine chorionicity is the first trimester&#46; As early as 4&#8211;5 weeks postmenstrual weeks&#44; by using transvaginal sonography&#44; it is possible to recognize the chorionic sacs inside the uterine cavity within the tick deciduas&#46; They appear as sonolucent round structures with a brightly echogenic rim&#46; By simply counting the chorionic sacs it is possible to establish whether the pregnancy will be monochorionic&#44; dichorionic&#44; trichorionic and so on &#40;<a class="elsevierStyleCrossRef" href="#fig0005">Fig&#46; 1</a>&#41;&#46;</p><elsevierMultimedia ident="fig0005"></elsevierMultimedia><p id="par0040" class="elsevierStylePara elsevierViewall">The visualization of a single chorionic sac at 4&#8211;5 weeks&#44; however&#44; does not exclude the possibility of a monochorionic twin pregnancy&#46; By the sixth postmenstrual week the yolk sac and the embryo can be visualized inside the chorionic sac and the definitive diagnosis of a single pregnancy or monochorionic twin pregnancy can be done&#46; If a single chorionic sac is visualized containing two yolk sacs and two embryonic poles&#44; then a monochorionic twin pregnancy can be diagnosed &#40;<a class="elsevierStyleCrossRef" href="#fig0010">Fig&#46; 2</a>&#41;&#46;</p><elsevierMultimedia ident="fig0010"></elsevierMultimedia><p id="par0045" class="elsevierStylePara elsevierViewall">Determination of amnioticity is difficult until 8 weeks of gestation&#46; Before that time the amnion is still too close to the embryo and is identifiable in only 50&#37; of the cases&#46;<a class="elsevierStyleCrossRef" href="#bib0045"><span class="elsevierStyleSup">9</span></a> The amnions appear as thin echogenic membranes surrounding the two embryos inside the celomatic cavity &#40;<a class="elsevierStyleCrossRef" href="#fig0015">Fig&#46; 3</a>&#41;&#46;</p><elsevierMultimedia ident="fig0015"></elsevierMultimedia><p id="par0050" class="elsevierStylePara elsevierViewall">Lack of visualization of the amniotic membranes after 8 weeks allows the diagnosis of monochorionic monoamniotic twins&#46; In this case the number of yolk sac seen may be one or two depending on the timing of cell division&#46;<a class="elsevierStyleCrossRef" href="#bib0050"><span class="elsevierStyleSup">10</span></a></p><p id="par0055" class="elsevierStylePara elsevierViewall">After 10 weeks of gestation chorionicity is determined by the evaluation of the number of the placental masses and&#44; in case of a single placental mass&#44; by the characteristics of the membrane dividing the amniotic sacs&#46; In dichorionic twins with fused placentas&#44; amnios and chorion reflect away from the placental surface&#44; creating a potential space into which villi can grow&#46; Sonographically this space appears as a triangular-shaped extension of placental tissue into the base on the inter-twin membrane&#46; This sign was first described in 1981 as the &#8220;lambda&#8221; sign<a class="elsevierStyleCrossRef" href="#bib0055"><span class="elsevierStyleSup">11</span></a> and is used interchangeably with the &#8220;twin peak&#8221; sign described in 1992<a class="elsevierStyleCrossRef" href="#bib0060"><span class="elsevierStyleSup">12</span></a> &#40;<a class="elsevierStyleCrossRef" href="#fig0020">Fig&#46; 4</a>&#41;&#46; In monochorionic pregnancy there is a single layer of continuous chorion limiting the villous growth&#59; the intertwin membrane takes off perpendicularly to the placental surface producing the sonographic sign known as &#8220;T&#8221; sign &#40;<a class="elsevierStyleCrossRef" href="#fig0025">Fig&#46; 5</a>&#41;&#46;</p><elsevierMultimedia ident="fig0020"></elsevierMultimedia><elsevierMultimedia ident="fig0025"></elsevierMultimedia><p id="par0060" class="elsevierStylePara elsevierViewall">Several studies have confirmed the accuracy of &#8220;lambda&#8221; and &#8220;T&#8221; signs in predicting chorionicity in the first trimester&#44; comparing the sonographic diagnosis with the placental hystology or the discordant sex at birth&#46;<a class="elsevierStyleCrossRefs" href="#bib0065"><span class="elsevierStyleSup">13&#8211;17</span></a> The sensitivity reported ranges from 89&#44;8&#37; to 100&#37;&#59; the specificity ranges from 97&#46;4&#37; to 99&#46;8&#37; &#40;<a class="elsevierStyleCrossRef" href="#tbl0005">Table 1</a>&#41;&#46; The gestational age at the time of the sonographic examination varied in the different studies&#46; Stenhouse et al&#46;<a class="elsevierStyleCrossRef" href="#bib0065"><span class="elsevierStyleSup">13</span></a> and Menon et al&#46;<a class="elsevierStyleCrossRef" href="#bib0075"><span class="elsevierStyleSup">15</span></a>report a sensitivity of 100&#37; but they examined the patients at 7&#8211;14 and 5&#8211;14 weeks respectively&#44; thus including in these good results also the patients examined in the earliest stages of pregnancy &#40;below 10 weeks&#41; when the sonographic diagnosis relies on the number of the chorionic sacs rather then the T&#47;lambda sign&#46; The study by Lee et al&#46;<a class="elsevierStyleCrossRef" href="#bib0080"><span class="elsevierStyleSup">16</span></a>reported four sex discordant twin pairs with monochorionic placentas&#46; They used this finding to contest the belief that monochorionicity is synonymous with monozygosity&#46; However they did not perform genetic tests to confirm zygosity&#46; Carroll et al&#46;<a class="elsevierStyleCrossRef" href="#bib0175"><span class="elsevierStyleSup">14</span></a> compared the accuracy of T&#47;lambda sign and membrane thickness and found that membrane thickness improved sensitivity by 1&#46;3&#37;&#44; although its assessment is time consuming and highly inter- and intraobserver dependent&#46; The largest series is by Dias et al&#46;<a class="elsevierStyleCrossRef" href="#bib0085"><span class="elsevierStyleSup">17</span></a>&#58; they examined 613 twin pregnancies with transabdominal ultrasound at the time of the routine first trimester screening &#40;11&#8211;14 weeks&#41;&#58; sensitivity and specificity were 100&#37; and 99&#46;8&#37; respectively&#46; The only error occurred in a pregnancy complicated by a small hematoma running along the site of the membrane insertion&#46;</p><elsevierMultimedia ident="tbl0005"></elsevierMultimedia><p id="par0065" class="elsevierStylePara elsevierViewall">Another technique to assess chorionicity in the first trimester is the evaluation of the number of layers of the intertwin membrane&#44; considering that the dividing membrane is made by only two layers of amnios in monochorionic and four layers &#40;two amnios and two chorions&#41; in dichorionic twins&#46; This technique&#44; mainly used in the second and third trimester&#44; has also been applied in the first trimester using transvaginal approach and depicting the amnios and chorion at the membrane to placenta insertion&#46;<a class="elsevierStyleCrossRef" href="#bib0090"><span class="elsevierStyleSup">18</span></a></p><p id="par0070" class="elsevierStylePara elsevierViewall">In conclusion the accuracy of ultrasound in the assessment of chorionicity in the first trimester is high and is mainly based on the evaluation of lambda and T-sign&#46;</p><p id="par0075" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleItalic"><span class="elsevierStyleBold">Second and third trimester&#46;</span></span> Determination of chorionicity in the second and third trimester is more problematic and the diagnostic accuracy is lower than in the first trimester&#46; The diagnosis relies on the evaluation of fetal gender&#44; number of the placental masses and characteristics of the intertwin membrane&#46;<a class="elsevierStyleCrossRef" href="#bib0095"><span class="elsevierStyleSup">19</span></a></p><p id="par0080" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleUnderline">Fetal gender</span> The identification of discordant fetal gender indicates dichorionic twinning &#40;<a class="elsevierStyleCrossRef" href="#fig0030">Fig&#46; 6</a>&#41;&#46; The positive predictive value of discordant gender &#40;when correctly identified&#41; is 100&#37;&#46; However&#44; considering that around 50&#37; of concordant sex twins are dichorionic&#44; the definition of chorionicity needs further sonographic signs&#46;</p><elsevierMultimedia ident="fig0030"></elsevierMultimedia><p id="par0085" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleUnderline">Number of the placental masses</span> The location of the placental masses has a limited value in the assessment of chorionicity&#46; Separate placental sites are an indicator of dichorionic twins &#40;<a class="elsevierStyleCrossRef" href="#fig0035">Fig&#46; 7</a>&#41;&#46; However widely separated placentas are found in only one-third of all twin gestations&#46; Furthermore two separated placental masses may be seen in the rare cases of a monochorionic placenta which is bilobed or has a succenturiate lobe&#46; For this reason the sensitivity of two placental sites is 32&#37; and the PPV is 97&#46;7&#37;&#46;<a class="elsevierStyleCrossRef" href="#bib0100"><span class="elsevierStyleSup">20</span></a> A single placental mass may be indication of both monochorionic and dichorionic twins with fused placentas &#40;<a class="elsevierStyleCrossRef" href="#fig0040">Fig&#46; 8</a>&#41;&#46;</p><elsevierMultimedia ident="fig0035"></elsevierMultimedia><elsevierMultimedia ident="fig0040"></elsevierMultimedia><p id="par0090" class="elsevierStylePara elsevierViewall">Given these limitations&#44; identifying chorionicity must rely on other sonographic parameters particularly when a single placental mass is seen&#46;</p><p id="par0095" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleUnderline">Take-off of the intertwin membrane</span> The ultrasonic visualization of the intertwin membrane origin &#40;take-off&#41; from the placental surface in the second and third trimester has a limited diagnostic value as compared to the first trimester&#46; The membrane take-off to recognize lambda and T-sign may be obscured by the fetus&#44; particularly in the third trimester and when the placenta is posterior&#46; Furthermore&#44; it must be taken into account that as the pregnancy progresses the lambda sign may disappear as the consequence of the progressive regression of the chorion frondosum to form chorion leave at the base of the intertwin membrane&#46; The lambda has been shown to disappear by the 20th week of gestation in about 7&#37; of dichorionic twins with fused placentas&#46;<a class="elsevierStyleCrossRef" href="#bib0105"><span class="elsevierStyleSup">21</span></a> This supports the conclusion that while the presence of the lambda sign indicates dichorionicity&#44; its absence does not always rule it out&#46;<a class="elsevierStyleCrossRef" href="#bib0110"><span class="elsevierStyleSup">22</span></a></p><p id="par0100" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleUnderline">Thickness of the inter-twin membrane</span> The inter-twin membrane thickness is greater in dichorionic twins because it is made of four layers &#40;two chorions and two amnios&#41; as compared to monochorionic twins&#44; whose membrane is made by two layers &#40;two amnios&#41; and consequently is thinner and hairlike&#46;</p><p id="par0105" class="elsevierStylePara elsevierViewall">The membrane thickness can be measured with high resolution ultrasound equipments&#46; The membrane should be imaged perpendicularly to the ultrasonic beam after magnification &#40;<a class="elsevierStyleCrossRef" href="#fig0045">Fig&#46; 9</a>&#41;&#46; The most reliable site for measurement with the most reproducibility is close to the placenta &#40;&#60;3<span class="elsevierStyleHsp" style=""></span>cm&#41;&#46;<a class="elsevierStyleCrossRef" href="#bib0115"><span class="elsevierStyleSup">23</span></a> The cut-off value firstly suggested to differentiate dichorionic from monochorionic twins is 2<span class="elsevierStyleHsp" style=""></span>mm&#46; Using that value Winn et al&#46;<a class="elsevierStyleCrossRef" href="#bib0120"><span class="elsevierStyleSup">24</span></a> report a diagnostic accuracy of 82&#37; for monochorionicity and 95&#37; for dichorionicity&#46; Bracero and Bryne<a class="elsevierStyleCrossRef" href="#bib0125"><span class="elsevierStyleSup">25</span></a> report a sensitivity of 75&#46;7&#37;&#44; a specificity of 85&#46;7&#37;&#44; a positive predictive value of 96&#46;6&#37;&#44; and a negative predictive value of 40&#46;0&#37;&#44; for determining dichorionicity&#46; A lower cut-off of 1&#44;5<span class="elsevierStyleHsp" style=""></span>mm has been suggested for measurements made in the first trimester&#46;<a class="elsevierStyleCrossRef" href="#bib0175"><span class="elsevierStyleSup">14</span></a>The sensitivity of membrane thickness for determining chorionicity declines with increasing gestation with a sensitivity of only 52&#37; for dichorionicity in the third trimester&#46;<a class="elsevierStyleCrossRef" href="#bib0130"><span class="elsevierStyleSup">26</span></a></p><elsevierMultimedia ident="fig0045"></elsevierMultimedia><p id="par0110" class="elsevierStylePara elsevierViewall">In order to improve the reproducibility of the membrane thickness measurement and to be sure that the membrane is perpendicular to the ultrasonic beam&#44; three dimensional multiplanar sonographic evaluation has been suggested with a very slight increase of sensitivity&#46;<a class="elsevierStyleCrossRef" href="#bib0135"><span class="elsevierStyleSup">27</span></a></p><p id="par0115" class="elsevierStylePara elsevierViewall">Due to these limitations&#44; to the technical difficulties and to the operator dependency&#44; the measurement of the inter-twin membrane thickness is of limited value in determining chorionicity in the second and third trimesters&#46;</p><p id="par0120" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleUnderline">Number of the layers of the inter-twin membrane</span> The inter-twin membrane is made by four layers &#40;amnios-chorion-chorion-amnios&#41; in dichorionic and only two layers &#40;amnios-amnios&#41; in monochorionic twins&#46; Using high resolution equipment and magnified images with the membrane perpendicular to the ultrasound beam it is possible to recognize the layers&#59; counting more than two layers allows to diagnose dichorionicity<a class="elsevierStyleCrossRefs" href="#bib0095"><span class="elsevierStyleSup">19&#8211;29</span></a> &#40;<a class="elsevierStyleCrossRef" href="#fig0045">Fig&#46; 9</a>&#41;&#46; It is useful to visualize the membrane close to its placental insertion in order to minimize the risk of error due to the folding back of a monochorionic membrane on itself thus appearing to have four layers&#46;</p><p id="par0125" class="elsevierStylePara elsevierViewall">By counting the number of layers in the inter-twin membrane a 100&#37; predictive accuracy for dichorionic and 94&#44;4&#37; for monochorionic have been reported&#46;<a class="elsevierStyleCrossRef" href="#bib0145"><span class="elsevierStyleSup">29</span></a> Another study reported a PPV of 100&#37; for dichorionicity&#46;<a class="elsevierStyleCrossRef" href="#bib0150"><span class="elsevierStyleSup">30</span></a> The technique is more accurate in the second rather than the third trimester due to the tinning of the membrane with progressing gestational age&#46;<a class="elsevierStyleCrossRef" href="#bib0155"><span class="elsevierStyleSup">31</span></a></p><p id="par0130" class="elsevierStylePara elsevierViewall">Counting of the layers in the inter-twin membrane is not commonly used in the clinical practice&#44; because it is time consuming&#44; technically difficult and operator dependent&#46;</p><p id="par0135" class="elsevierStylePara elsevierViewall"><span class="elsevierStyleUnderline">Doppler evaluation of the placenta</span> A further technique suggested to recognize monochorionic twins in the third trimester is the detection of arterio-arterial anastomoses using Doppler evaluation of the placenta&#46; Using this technique Fichera et al&#46;<a class="elsevierStyleCrossRef" href="#bib0160"><span class="elsevierStyleSup">32</span></a> report a sensitivity of 75&#37; and a specificity of 100&#37; with detection rate increasing with gestational age&#46;</p><p id="par0140" class="elsevierStylePara elsevierViewall">Also this technique&#44; however&#44; is time consuming&#44; operator dependent and requires highly sophisticated equipments and its use in determining chorionicity in the clinical practice is very limited&#46;</p></span><span id="sec0020" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0040">Conclusions</span><p id="par0145" class="elsevierStylePara elsevierViewall">The prenatal determination of chorionicity is the first step for an accurate managing of twin gestation&#46; Knowledge of chorionicity helps in risk assessment&#44; genetic counseling&#44; invasive procedure and management of TTTS and selective IUGR&#44; death of one twin and discordant fetal anomaly&#46; It is best done in the first trimester&#44; when the diagnostic accuracy approaches 100&#37;&#46; The most reliable sonographic signs are the lambda and T-sign and the number of the placental masses evaluated before 14 weeks of gestation&#46;</p><p id="par0150" class="elsevierStylePara elsevierViewall">Despite the widespread diffusion of ultrasound as a basic screening test&#44; there is still limited knowledge about the correct diagnosis of chorionicity&#46; A recent study showed that only 43&#37; of multifetal pregnancies carried an accurate diagnosis of chorionicity before tertiary care center evaluation&#46;<a class="elsevierStyleCrossRef" href="#bib0165"><span class="elsevierStyleSup">33</span></a> For this reason if there is uncertainty in the diagnosis of chorionicity the patient should be referred to a specialist&#46;</p><p id="par0155" class="elsevierStylePara elsevierViewall">The importance of a correct diagnosis of chorionicity in twin gestation has been recently stressed by the report of the National Collaborating Center for Women&#39;s and Children&#39;s Health on Multiple Pregnancy&#44;<a class="elsevierStyleCrossRef" href="#bib0170"><span class="elsevierStyleSup">34</span></a> whose recommendations are&#58;<ul class="elsevierStyleList" id="lis0005"><li class="elsevierStyleListItem" id="lsti0005"><span class="elsevierStyleLabel">&#8226;</span><p id="par0160" class="elsevierStylePara elsevierViewall">Determine chorionicity at the time of detecting twin pregnancy by ultrasound using the number of placental masses&#44; the lambda and T-sign and membrane thickness&#46;</p></li><li class="elsevierStyleListItem" id="lsti0010"><span class="elsevierStyleLabel">&#8226;</span><p id="par0165" class="elsevierStylePara elsevierViewall">Assign nomenclature to fetuses &#40;for example&#58; upper and lower&#44; left and right&#41; and document this clearly in the woman&#39;s notes&#46;</p></li><li class="elsevierStyleListItem" id="lsti0015"><span class="elsevierStyleLabel">&#8226;</span><p id="par0170" class="elsevierStylePara elsevierViewall">If a woman with a twin pregnancy presents after 14 weeks determine chorionicity at the earliest opportunity by using all the possible signs &#40;number of placental masses&#44; lambda and T-sign&#44; membrane thickness&#44; discordant sex&#41;&#46;</p></li><li class="elsevierStyleListItem" id="lsti0020"><span class="elsevierStyleLabel">&#8226;</span><p id="par0175" class="elsevierStylePara elsevierViewall">If it is not possible to determine chorionicity by ultrasound at the time of detecting the twin pregnancy&#44; seek a second opinion from a senior ultrasonographer or offer the woman referral to a healthcare professional who is competent in determining chorionicity as soon as possible&#46;</p></li><li class="elsevierStyleListItem" id="lsti0025"><span class="elsevierStyleLabel">&#8226;</span><p id="par0180" class="elsevierStylePara elsevierViewall">If it is difficult to determine chorionicity even after referral manage the pregnancy as monochorionic until proved otherwise&#46;</p></li><li class="elsevierStyleListItem" id="lsti0030"><span class="elsevierStyleLabel">&#8226;</span><p id="par0185" class="elsevierStylePara elsevierViewall">Provide regular training so that ultrasonographers can identify the lambda and T-sign accurately and confidently&#46; Training should cover ultrasound scan measurements needed for women who book after 14 weeks and should emphasize that the risks associated with twin pregnancies are determined by chorionicity and not zygosity&#46;</p></li><li class="elsevierStyleListItem" id="lsti0035"><span class="elsevierStyleLabel">&#8226;</span><p id="par0190" class="elsevierStylePara elsevierViewall">Conduct regular clinical audits to evaluate the accuracy of determining chorionicity&#46;</p></li></ul></p></span><span id="sec0025" class="elsevierStyleSection elsevierViewall"><span class="elsevierStyleSectionTitle" id="sect0045">Conflict of interest</span><p id="par0195" class="elsevierStylePara elsevierViewall">The authors declare no conflict of interest&#46;</p></span></span>"
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          "titulo" => "Abstract"
        ]
        1 => array:2 [
          "identificador" => "xpalclavsec362440"
          "titulo" => "Keywords"
        ]
        2 => array:2 [
          "identificador" => "xres383469"
          "titulo" => "Resumen"
        ]
        3 => array:2 [
          "identificador" => "xpalclavsec362439"
          "titulo" => "Palabras clave"
        ]
        4 => array:2 [
          "identificador" => "sec0005"
          "titulo" => "Introduction"
        ]
        5 => array:2 [
          "identificador" => "sec0010"
          "titulo" => "Zygosity and chorionicity"
        ]
        6 => array:2 [
          "identificador" => "sec0015"
          "titulo" => "Ultrasound diagnosis of chorionicity"
        ]
        7 => array:2 [
          "identificador" => "sec0020"
          "titulo" => "Conclusions"
        ]
        8 => array:2 [
          "identificador" => "sec0025"
          "titulo" => "Conflict of interest"
        ]
        9 => array:1 [
          "titulo" => "References"
        ]
      ]
    ]
    "pdfFichero" => "main.pdf"
    "tienePdf" => true
    "fechaRecibido" => "2013-08-28"
    "fechaAceptado" => "2013-09-12"
    "PalabrasClave" => array:2 [
      "en" => array:1 [
        0 => array:4 [
          "clase" => "keyword"
          "titulo" => "Keywords"
          "identificador" => "xpalclavsec362440"
          "palabras" => array:3 [
            0 => "Twins"
            1 => "Chorionicity"
            2 => "Ultrasound"
          ]
        ]
      ]
      "es" => array:1 [
        0 => array:4 [
          "clase" => "keyword"
          "titulo" => "Palabras clave"
          "identificador" => "xpalclavsec362439"
          "palabras" => array:3 [
            0 => "Gemelos"
            1 => "Corionicidad"
            2 => "Ecograf&#237;a"
          ]
        ]
      ]
    ]
    "tieneResumen" => true
    "resumen" => array:2 [
      "en" => array:2 [
        "titulo" => "Abstract"
        "resumen" => "<p id="spar0005" class="elsevierStyleSimplePara elsevierViewall">Chorionicity is the main determinant of the perinatal outcome in twin pregnancies&#58; perinatal mortality and morbidity are significantly higher in monochorionic versus dichorionic twins&#46; This is mainly due to complications associated specifically with monochorionicity&#44; such as twin to twin transfusion syndrome &#40;TTTS&#41;&#44; selective fetal growth restriction &#40;FGR&#41; and twin reverse arterial perfusion syndrome &#40;TRAP&#41;&#44; consequences of the presence of inter-twin vascular anastomoses in the common placenta&#46; For this reason the diagnosis of chorionicity in twins is of clinical importance in order to plan an increased surveillance in monochorionic gestations and to recognize the appearance of complications in their early stages&#46;</p><p id="spar0010" class="elsevierStyleSimplePara elsevierViewall">Different sonographic signs may be used to evaluate chorionicity&#58; number of placental masses&#44; sex of the fetuses&#44; characteristics of the intertwine membrane&#46; The last one is surely the most useful and valuable tool&#58; the take-off of the membrane from the placental surface shows the typical &#8220;lambda&#8221; appearance in dichorionic pregnancy and the typical &#8220;T&#8221; appearance in the monochorionic ones&#46; In this article the sonographic features that help in the accurate depiction of chorionicity are reviewed&#46;</p>"
      ]
      "es" => array:2 [
        "titulo" => "Resumen"
        "resumen" => "<p id="spar0015" class="elsevierStyleSimplePara elsevierViewall">La corionicidad es el principal determinante del desenlace perinatal en los embarazos gemelares&#58; la morbimortalidad perinatal es significativamente superior en los gemelos monocori&#243;nicos que en los bicori&#243;nicos&#46; Esto se debe principalmente a las complicaciones asociadas con la corionicidad&#44; a saber&#44; el s&#237;ndrome de transfusi&#243;n fetal-fetal &#40;STFF&#41;&#44; la restricci&#243;n del crecimiento fetal selectivo &#40;CIR&#41; y la perfusi&#243;n arterial reversa &#40;secuencia TRAP&#41;&#44; que son debidas a anastomosis vasculares intergemelares en la placenta com&#250;n&#46; Por esta raz&#243;n&#44; el diagn&#243;stico de corionicidad en gemelos es de importancia cl&#237;nica&#44; ya que as&#237; se podr&#225; planificar una mayor vigilancia en gestaciones monocori&#243;nicas y detectar la aparici&#243;n de complicaciones en las primeras etapas&#46;</p><p id="spar0020" class="elsevierStyleSimplePara elsevierViewall">Distintos signos ecogr&#225;ficos sirven para evaluar la corionicidad&#58; n&#250;mero de masas placentarias&#44; sexo de los fetos&#44; caracter&#237;sticas de la membrana intergemelar&#46; Esto &#250;ltimo es&#44; sin duda&#44; la herramienta m&#225;s &#250;til y valiosa&#58; la separaci&#243;n de la membrana de la superficie placentaria muestra una t&#237;pica forma &#171;lambda&#187; en el embarazo bicori&#243;nico y un t&#237;pico signo en &#171;T&#187; en el monocori&#243;nico&#46; En este art&#237;culo se revisan las caracter&#237;sticas ecogr&#225;ficas que ayudan a la descripci&#243;n exacta de la corionicidad&#46;</p>"
      ]
    ]
    "multimedia" => array:10 [
      0 => array:7 [
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        "etiqueta" => "Fig&#46; 1"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
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        "descripcion" => array:1 [
          "en" => "<p id="spar0025" class="elsevierStyleSimplePara elsevierViewall">Dichorionic twin pregnancy at 5 weeks 3 days&#46; Two round sonolucent sacs with a brightly echogenic rim are clearly visible in the thick decidua&#46;</p>"
        ]
      ]
      1 => array:7 [
        "identificador" => "fig0010"
        "etiqueta" => "Fig&#46; 2"
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        "figura" => array:1 [
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          "en" => "<p id="spar0030" class="elsevierStyleSimplePara elsevierViewall">At 6 postmenstrual weeks a single chorionic sac is seen containing two yolk sacs&#58; the diagnosis of monochorionic twin pregnancy can be done&#59; it is not possible yet to diagnose amnioticity&#46;</p>"
        ]
      ]
      2 => array:7 [
        "identificador" => "fig0015"
        "etiqueta" => "Fig&#46; 3"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
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        "descripcion" => array:1 [
          "en" => "<p id="spar0035" class="elsevierStyleSimplePara elsevierViewall">Monochorionic diamniotic twin pregnancy at 8 postmenstrual weeks&#46; The two amnions appear as thin echogenic membranes &#40;arrows&#41; close to the two embryos&#44; inside the celomatic cavity&#46;</p>"
        ]
      ]
      3 => array:7 [
        "identificador" => "fig0020"
        "etiqueta" => "Fig&#46; 4"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
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        "figura" => array:1 [
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        "descripcion" => array:1 [
          "en" => "<p id="spar0040" class="elsevierStyleSimplePara elsevierViewall">Twin peak sign &#40;arrow&#41; in a dichorionic twin pregnancy in the first trimester&#46;</p>"
        ]
      ]
      4 => array:7 [
        "identificador" => "fig0025"
        "etiqueta" => "Fig&#46; 5"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
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        "descripcion" => array:1 [
          "en" => "<p id="spar0045" class="elsevierStyleSimplePara elsevierViewall">T-sign &#40;arrow&#41; in a monochorionic twin pregnancy in the first trimester&#46;</p>"
        ]
      ]
      5 => array:7 [
        "identificador" => "fig0030"
        "etiqueta" => "Fig&#46; 6"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
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        "descripcion" => array:1 [
          "en" => "<p id="spar0050" class="elsevierStyleSimplePara elsevierViewall">Dichorionic twin pregnancy in the second trimester&#46; Arrows point to the fetal genitalia &#40;female on the left and male on the right&#41;&#46;</p>"
        ]
      ]
      6 => array:7 [
        "identificador" => "fig0035"
        "etiqueta" => "Fig&#46; 7"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr7.jpeg"
            "Alto" => 491
            "Ancho" => 650
            "Tamanyo" => 56555
          ]
        ]
        "descripcion" => array:1 [
          "en" => "<p id="spar0055" class="elsevierStyleSimplePara elsevierViewall">Dichorionic twin pregnancy in the second trimester&#58; two widely separated placental masses &#40;P&#41; can be seen&#44; one on the anterior and one on the posterior wall of the uterus&#46;</p>"
        ]
      ]
      7 => array:7 [
        "identificador" => "fig0040"
        "etiqueta" => "Fig&#46; 8"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr8.jpeg"
            "Alto" => 837
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            "Tamanyo" => 99401
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        "descripcion" => array:1 [
          "en" => "<p id="spar0060" class="elsevierStyleSimplePara elsevierViewall">Twin pregnancy in the second trimester&#58; a single placental mass &#40;A&#41; is seen on the anterior wall of the uterus&#46; The chorionicity cannot be determined on the only base of the placental masses&#46;</p>"
        ]
      ]
      8 => array:7 [
        "identificador" => "fig0045"
        "etiqueta" => "Fig&#46; 9"
        "tipo" => "MULTIMEDIAFIGURA"
        "mostrarFloat" => true
        "mostrarDisplay" => false
        "figura" => array:1 [
          0 => array:4 [
            "imagen" => "gr9.jpeg"
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            "Ancho" => 1083
            "Tamanyo" => 60425
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        "descripcion" => array:1 [
          "en" => "<p id="spar0065" class="elsevierStyleSimplePara elsevierViewall">Evaluation of the inter-twin membrane in the second trimester&#46; The membrane is imaged perpendicularly to the ultrasonic beam and magnified&#46; &#40;A&#41; monochorionic twins&#58; two layers may be seen and the thickness is below 2<span class="elsevierStyleHsp" style=""></span>mm &#40;B&#41; dichorionic twins&#58; more than two layers may be seen and the thickness is above 2<span class="elsevierStyleHsp" style=""></span>mm&#46;</p>"
        ]
      ]
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                  <table border="0" frame="\n
                  \t\t\t\t\tvoid\n
                  \t\t\t\t" class=""><thead title="thead"><tr title="table-row"><td class="td" title="\n
                  \t\t\t\t\ttable-head\n
                  \t\t\t\t  " align="left" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t" style="border-bottom: 2px solid black">Author&nbsp;\t\t\t\t\t\t\n
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                  \t\t\t\t  " align="center" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t" style="border-bottom: 2px solid black">Pregnancies &#40;n&#41;&nbsp;\t\t\t\t\t\t\n
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                  \t\t\t\t\ttable-head\n
                  \t\t\t\t  " align="center" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t" style="border-bottom: 2px solid black">GA &#40;weeks&#41;&nbsp;\t\t\t\t\t\t\n
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                  \t\t\t\t\ttable-head\n
                  \t\t\t\t  " align="center" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t" style="border-bottom: 2px solid black">Sensitivity &#40;&#37;&#41;&nbsp;\t\t\t\t\t\t\n
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                  \t\t\t\t\ttable-head\n
                  \t\t\t\t  " align="center" valign="\n
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                  \t\t\t\t" style="border-bottom: 2px solid black">Specificity &#40;&#37;&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td></tr></thead><tbody title="tbody"><tr title="table-row"><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="left" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">Stenhouse &#40;2002&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">96&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">7&#8211;14&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">100&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">98&#46;7&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="left" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">Carrol &#40;2002&#41;&nbsp;\t\t\t\t\t\t\n
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                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">150&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">10&#8211;14&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">98&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">97&#46;4&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="left" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">Menon &#40;2005&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
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                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">344&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">5&#8211;14&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">100&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
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                  \t\t\t\t">97&#46;9&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td></tr><tr title="table-row"><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="left" valign="\n
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                  \t\t\t\t">Lee &#40;2006&#41;&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">247&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
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                  \t\t\t\t">&#60;14&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
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                  \t\t\t\t  " align="char" valign="\n
                  \t\t\t\t\ttop\n
                  \t\t\t\t">89&#44;8&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
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                  \t\t\t\t  " align="char" valign="\n
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                  \t\t\t\t">99&#46;5&nbsp;\t\t\t\t\t\t\n
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                  \t\t\t\t  " align="left" valign="\n
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                  \t\t\t\t">Dias &#40;2011&#41;&nbsp;\t\t\t\t\t\t\n
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                  \t\t\t\t  " align="char" valign="\n
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                  \t\t\t\t">613&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
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                  \t\t\t\t  " align="char" valign="\n
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                  \t\t\t\t">11&#8211;14&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
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                  \t\t\t\t">100&nbsp;\t\t\t\t\t\t\n
                  \t\t\t\t</td><td class="td" title="\n
                  \t\t\t\t\ttable-entry\n
                  \t\t\t\t  " align="char" valign="\n
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                  \t\t\t\t">99&#46;8&nbsp;\t\t\t\t\t\t\n
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              ]
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        "descripcion" => array:1 [
          "en" => "<p id="spar0070" class="elsevierStyleSimplePara elsevierViewall">Sensitivity and specificity of lambda and T signs for determining chorionicity in the first trimester of pregnancy&#46;</p>"
        ]
      ]
    ]
    "bibliografia" => array:2 [
      "titulo" => "References"
      "seccion" => array:1 [
        0 => array:2 [
          "identificador" => "bibs0005"
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            0 => array:3 [
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              "etiqueta" => "1"
              "referencia" => array:1 [
                0 => array:2 [
                  "contribucion" => array:1 [
                    0 => array:2 [
                      "titulo" => "Births&#58; final data for 2008"
                      "autores" => array:1 [
                        0 => array:2 [
                          "etal" => false
                          "autores" => array:6 [
                            0 => "J&#46;A&#46; Martin"
                            1 => "B&#46;E&#46; Hamilton"
                            2 => "P&#46;D&#46; Sutton"
                            3 => "S&#46;J&#46; Ventura"
                            4 => "T&#46;J&#46; Mathews"
                            5 => "M&#46;J&#46; Osterman"
                          ]
                        ]
                      ]
                    ]
                  ]
                  "host" => array:1 [
                    0 => array:1 [
                      "Revista" => array:5 [
                        "tituloSerie" => "Natal Vital Stat Rep"
                        "fecha" => "2010"
                        "volumen" => "59"
                        "paginaInicial" => "3"
                        "paginaFinal" => "71"
                      ]
                    ]
                  ]
                ]
              ]
            ]
            1 => array:3 [
              "identificador" => "bib0010"
              "etiqueta" => "2"
              "referencia" => array:1 [
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Article information

ISSN: 21734127
Original language: English

DOI:10.1016/j.diapre.2013.09.004

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Year/Month	Html	Pdf	Total

2024 October	930	21	951
2024 September	1573	25	1598
2024 August	1376	15	1391
2024 July	1262	32	1294
2024 June	1057	17	1074
2024 May	1003	14	1017
2024 April	1043	29	1072
2024 March	1158	21	1179
2024 February	1312	43	1355
2024 January	1706	27	1733
2023 December	1094	48	1142
2023 November	1209	64	1273
2023 October	1403	55	1458
2023 September	1062	36	1098
2023 August	915	24	939
2023 July	1069	45	1114
2023 June	924	29	953
2023 May	975	43	1018
2023 April	903	28	931
2023 March	938	22	960
2023 February	915	18	933
2023 January	856	16	872
2022 December	851	60	911
2022 November	1147	23	1170
2022 October	939	24	963
2022 September	996	31	1027
2022 August	847	43	890
2022 July	787	49	836
2022 June	845	39	884
2022 May	859	40	899
2022 April	778	40	818
2022 March	936	46	982
2022 February	847	35	882
2022 January	963	69	1032
2021 December	761	26	787
2021 November	868	35	903
2021 October	939	43	982
2021 September	1368	36	1404
2021 August	1151	53	1204
2021 July	912	37	949
2021 June	910	37	947
2021 May	803	40	843
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2021 January	769	37	806
2020 December	768	39	807
2020 November	670	22	692
2020 October	638	18	656
2020 September	1015	61	1076
2020 August	883	40	923
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2020 June	782	34	816
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2020 February	1108	57	1165
2020 January	1080	44	1124
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2019 July	600	47	647
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2019 February	282	18	300
2019 January	212	21	233
2018 December	136	9	145
2018 November	163	9	172
2018 October	273	13	286
2018 September	328	18	346
2018 August	359	4	363
2018 July	339	4	343
2018 June	367	3	370
2018 May	466	9	475
2018 April	354	3	357
2018 March	310	4	314
2018 February	222	4	226
2018 January	192	6	198
2017 December	190	14	204
2017 November	183	5	188
2017 October	172	8	180
2017 September	269	12	281
2017 August	183	8	191
2017 July	217	15	232
2017 June	228	27	255
2017 May	256	11	267
2017 April	183	6	189
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2017 February	307	9	316
2017 January	230	3	233
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2016 November	183	14	197
2016 October	198	16	214
2016 September	223	12	235
2016 August	251	11	262
2016 July	276	13	289
2016 June	397	1	398
2016 May	367	6	373
2016 April	288	1	289
2016 March	329	41	370
2016 February	250	31	281
2016 January	207	34	241
2015 December	186	30	216
2015 November	215	34	249
2015 October	151	42	193
2015 September	96	30	126
2015 August	58	31	89
2015 July	38	22	60
2015 June	1	0	1
2015 May	0	1	1
2014 December	0	1	1
2014 November	0	2	2

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