Determination of chorionicity in twin gestations by high-frequency abdominal ultrasonography: counting the layers of the dividing membrane

Screening and Testing in Multiples

The choice of screening or invasive procedure in twin pregnancies is a personal choice of whether the patient wishes to take a small risk of having a baby with a serious disorder versus a small risk of having a complication because she wishes to avoid that. How to interpret such risks has profound effects on the perceived value of techniques, either leading to a decision to screening or going directly to chorionic villus sampling. There are profound issues surrounding the data and the interpretation of the data. No single short review can exhaustively examine all of the issues.

Ultrasound Determination of Chorionicity in Twin Pregnancy: Accuracy and Operator Experience

To evaluate the accuracy of ultrasound in early detection of chorionicity and amnionicity in twin pregnancies and evaluate the role of operator-experience, a prospective follow up of 87 consecutive twin pregnancies was performed. Chorionicity and amnionicity were determined by 17 different observers, divided in 2 groups on the basis of their experience, using a composite of US markers at the first US examination confirming a viable intrauterine twin pregnancy. Accuracy of prediction was determined for both groups by comparison with postnatal pathology. From the 87 pregnancies examined at 10.1 (minimum 5.5 weeks, maximum 26.0) weeks of gestation, pathology was available in 82 cases and a correct prediction of chorionicity was made in all but two cases, both being diagnosed as monochorionic while dichorionic by observers belonging the more experienced group. No monochorionic pregnancy was missed and amnionicity was correctly diagnosed in all cases, including two monoamniotic twins. We conclude that use of a composite of ultrasound markers for early detection of chorionicity and amnionicity is reliable in a clinical setting where physicians with a variable degree of experience perform ultrasound examination.

The Frequency and Clinical Significance of Twin Gestations According to Zygosity and Chorionicity

Background:Although the clinical importance of chorionicity in twin pregnancies has been studied widely, the significance of perinatal determination of zygosity using molecular genetic analyses remains controversial. The purpose of this study was to determine the frequency and clinical significance of twin gestations according to zygosity and chorionicity in a Korean population.Methods:We enrolled 569 women who delivered twin newborns (> 24 weeks) at Seoul National University Hospital between 1999 and 2008. Chorionicity was established by histologic examination of placentae. Zygosity was determined with sex of neonates, chorionicity, and DNA analysis of umbilical cord blood.Results:The frequency of dizygotic (DZ) twins was 71.0% (404/569 pairs) based on the opposite sex (238/404 [58.9%]) and DNA analyses (166/404 [41.1%]); that of monozygotic (MZ) twins was 29.0% (165/569), including monochorionic (MC) (72.1% [119/165]) and dichorionic (DC) twins (27.9% [46/165]), which was confirmed by DNA analyses. Among spontaneously conceived twins, the frequency of MZ twins was more than twice that of DZ twins. The risk of low birth weight was 1.8-fold higher among MZDC twins and 1.9-fold higher among MZMC twins than among DZDC twins (p< .05). Bronchopulmonary dysplasia occurred more frequently among MZMC twins than among DZDC twins (adjusted OR 8.42, 95% CI 1.82–39.08,p< .01). However, the frequencies of other neonatal morbidities were not significantly higher in the MZMC group than in the MZDC and DZDC groups. The perinatal mortality rate was 15 per 1000 total births in the DZDC twins, 20 per 1000 total births in the MZDC and 56 per 1000 total births in the MZMC (p< .01).Conclusions:Although monozygosity was shown to be a risk factor for perinatal death and accurate determination of zygosity plays a great role in the future consideration of organ transplantation and twin studies, the value of zygosity determination along with chorionicity in relation to overall neonatal morbidity was not definite.

Determining chorionicity in twin gestations: three-dimensional (3D) multiplanar sonographic measurement of intra-amniotic membrane thickness

OBJECTIVES

Measuring the thickness of the membranes to determine chorionicity in twins using two-dimensional (2D) ultrasound may provide uncertain results, given the possibility of measuring in an oblique view. We aimed to determine whether chorionicity can be assessed during the second and third trimesters by measuring the thickness of the membranes with three-dimensional (3D) multiplanar ultrasound, which ensures the measurement is perpendicular to the membranes, and to evaluate the reproducibility of the technique.

METHODS

This was a prospective study of 84 twin pregnancies (30 monochorionic and 54 dichorionic) between 20 and 35 weeks of gestation. The dividing membranes between the fetuses were visualized with 2D and 3D transabdominal sonography and the thickness of the membranes was measured by the multiplanar technique. Intraobserver variability was assessed by comparing the measurements obtained in sagittal and axial views by the same observer. We examined the frequency distributions to determine the optimal cut-off point of the thickness of the membranes to predict chorionicity. Kappa index, intraclass correlation coefficients and 95% limits of agreement were calculated to evaluate the inter- and intraoperator variability. We also compared membrane-thickness measurement, during the second and third trimesters, using 2D and 3D ultrasound.

RESULTS

In monochorionic and dichorionic pregnancies, respectively, the mean thickness was 1.42 (SD, 0.31) mm and 2.48 (SD, 0.47) mm. With 3D ultrasound, the intraclass correlation coefficient was 0.99 in all cases. The 95% limits of agreement were all within +/-0.2 mm. The best cut-off for membrane thickness for discriminating monochorionic from dichorionic twinning was 1.8 mm. Using this cut-off, there were one false-negative (sensitivity, 97%; 95% CI, 83-99%) and three false-positive (specificity, 94%; 95% CI, 85-99%) diagnoses of monochorionic twins. Kappa indices for intra- and interoperator variability were all above 90%, suggesting almost perfect agreement. Measurements with 2D sonography were less accurate (sensitivity and specificity, 83%).

CONCLUSION

Prenatal assessment of dividing membranes using 3D multiplanar ultrasound is reproducible and may be a useful alternative method for determining chorionicity in twin pregnancies during the second and third trimesters.

Antenatal sonographic prediction of twin chorionicity

OBJECTIVE

We sought to determine the accuracy of antenatal diagnosis of twin chorionicity at a single tertiary care center and assess the consequences of incorrect diagnoses.

STUDY DESIGN

Twins with chorionicity diagnosed by ultrasound < or = 24 weeks' gestation were retrospectively reviewed. Chorionicity was assigned by sonographic findings including placental location(s), the lambda and T-signs, and/or fetal gender(s). Postnatal diagnosis was determined by placental histopathologic examination. Medical records of antenatal-postnatal discordant chorionicities were reviewed for adverse sequelae.

RESULTS

Chorionicity was correctly assigned antenatally in 392/410 (95.6%) twins. The sensitivity, specificity, and positive and negative predictive values of monochorionicity assessed < or = 14 weeks were 89.8%, 99.5%, 97.8%, and 97.5%. Corresponding statistical values for the second trimester were 88.0%, 94.7%, 88.0%, and 94.7%. Two cases of inaccurate antenatal diagnoses affected patient counseling or were associated with adverse clinical outcomes.

CONCLUSION

Antenatal assessment of chorionicity is accurate; however, incorrect diagnoses do occur and can affect reliable patient counseling and management.

The management of multiple pregnancy

The epidemic of multiple pregnancy continues albeit in a different form with twin pregnancies predominating. Determination of chorionicity is the key to management and regular monitoring by ultrasound is a hallmark of quality care. All multiple pregnancies should be offered first trimester screening by nuchal translucency for aneuploidy. MC twins should be scanned at fortnightly intervals to allow complications such as twin-twin transfusion or IUGR to be detected and referral made to a fetal medicine centre. Maternal complications are common and vigilance is required for their detection. Although planned vaginal delivery can often be achieved, mothers must be prepared for the substantial risk of requiring a caesarean section as this currently occurs in the majority.

An association of chorionicity with preterm twin birth

OBJECTIVE

To assess the risk factors for preterm birth in twin pregnancies, particularly monochorionicity.

METHODS

A cohort study of 767 sets of twins, each twin weighing more than 500 g, born between January 1, 1992, and December 31, 2001, at St. Joseph's Health Care in London, Ontario. Statistical analysis was performed using forward stepwise logistic regression models, with gestational age at birth less than 28 or 32 weeks as the outcome.

RESULTS

Polyhydramnios and chorioamnionitis were significant risk factors for preterm birth prior to 28 or 32 weeks' gestation. Monochorionicity was a risk factor for preterm birth prior to 32 weeks' gestation. Past term birth and maternal age over 30 years were associated with reduced risk for preterm birth.

CONCLUSION

Monochorionic placentation is a significant risk factor for preterm twin birth.

Invasive Genetic Diagnosis in Multiple Pregnancies

Chromosomal anomalies and mendelian diseases are more frequent in multiple gestations than in singletons. Prenatal diagnosis is recommended in multiple pregnancies whenever indicated. Invasive testing using amniocentesis or chorionic villus sampling can be performed safely. Chorionic villus sampling has a significant advantage over amniocentesis because it offers rapid karyotyping and biochemical and DNA studies at an early stage of pregnancy. Only experienced centers should perform these procedures because of the technical aspects and the expertise needed in handling discordant results.

The sonographic diagnosis of chorionicity

The differentiation between mono- and dichorionic placentation in twin pregnancies is of clinical importance because of the significant difference in perinatal morbidity and mortality between the two, and the increased surveillance indicated in monochorionic gestations. Application of ultrasonography has enabled very precise prenatal determination of chorionicity. While this is best performed in the first trimester when accuracy approaches 100%, even in the third trimester, using a composite cascade of available sonographic features, accuracy has been reported to approach 97%. While two clearly separate placentae or discordant fetal gender conform to dichorionicity, in most twin pregnancies other features need to be assessed to determine chorionicity. The presence of the 'lambda' or the 'T' sign in the presence of a single placenta, best determined in the first trimester, is the most reliable indicator of chorionicity, with measurements of the inter-twin membrane thickness and counting of the membrane layers being less reliable. In this article, we review the sonographic features that help in the accurate depiction of chorionicity.

Invasive diagnostic procedures in twin pregnancies

The rising rate of multiple pregnancies and its association with advanced maternal age has expanded the need for prenatal diagnosis in twins and higher order gestations. The complexity of the invasive diagnostic procedures and the risk of loss of an unaffected twin raise significant clinical, technical and ethical issues. In this review we discuss the specific issues of early scanning, counseling and determination of chorionicity prior to invasive procedures in twins. We present the available data describing the risk associated with these procedures in twins and compare data of fetal loss rate from different studies. We also discuss the issues of fetal blood sampling and late karyotyping in twin pregnancies.

[Ultrasonic diagnosis of chorionicity in multiple pregnancies]

The management of multiple pregnancies requires a correct diagnosis of chorionicity. This assessment is easy and reliable before 15 weeks of gestation by ultrasonography, but becomes much more difficult during the second and third trimester for same sex foetuses. From 5 to 8 weeks of gestation, the visualization of two gestational sacs assesses bichorionicity. From 8 to 14 weeks, the diagnosis of chorionicity is based on the presence of the lambda sign, completed by the evaluation of the thickness of the inter-twin membrane and the placenta localisation. From 15 weeks onward, the twin peak (or lambda) sign remains the best predictor of dichorionicity but is valuable only if it is present. The measurement of the thickness of the inter-twin membrane and the count of its layers are not available in all cases. From March 1999 to March 2000, we studied 31 multiple pregnancies with same sex foetuses, referred to our centre during the second trimester. The patients were asked for their former ultrasound reports. Chorionicity was mentioned only in 77% of the cases and when mentioned the information was correct in 85% of the cases. Thus, improving on that point is necessary. Prospective studies focusing on ultrasound determination of chorionicity show accuracy close to 100% when the ultrasonography is correctly realized before 15 weeks of gestation.

Prediction of chorionicity in twin pregnancies at 10-14 weeks of gestation

OBJECTIVE

To examine the accuracy of sonographic determination of chorionicity in twin pregnancies at 10-14 weeks of gestation.

DESIGN

Prospective study on the sonographic prediction of chorionicity at 10-14 weeks of gestation.

PARTICIPANTS

During a 30 month period, from October 1997 to May 2000, 165 women attending the departments of fetal medicine or ultrasound.

METHODS

Sonographic criteria used in the diagnosis of chorionicity were the number of placental sites, the lambda (lambda) and T signs and the thickness of the inter-twin membrane. The diagnosis of chorionicity was made at the time of the ultrasound examination using all these features and subsequently compared with the postnatal diagnosis, confirmed either by placental histology or discordancy in infant sex.

RESULTS

In 150 cases with confirmation of chorionicity following delivery, 116 were postnatally classified as dichorionic and 34 monochorionic. Prenatal ultrasound examination correctly identified chorionicity in 149 (99.3%) cases. The most reliable indicator for dichorionicity was a combination using the lambda sign or two separate placentae with a sensitivity and specificity of 97.4% and 100%, respectively. The most useful test in predicting monochorionicity was the T sign with a sensitivity of 100% and specificity of 98.2%. Measurement of the inter-twin membrane thickness was a less reliable indicator where the sensitivity for dichorionicity and specificity for monochorionicity was only 92.6%.

CONCLUSIONS

Ultrasound examination of twin pregnancies at 10-14 weeks of gestation predicts chorionicity with a high degree of accuracy using a combination of the number of placentae, lambda and T signs and inter-twin membrane thickness. All hospitals should encourage departments providing ultrasound services to undertake chorionicity determination when examining women with twin pregnancies at this gestation.

Prenatal diagnosis of heterokaryotypic mosaic twins discordant for fetal sex

The presence of a monozygotic twin gestation with discordant sex of the twins is a very rare constellation, which is referred to as heterokaryotypic monozygotic pregnancy. This constellation can develop either due to a chromosomal aberration after twinning or is - as in the following case - due to a mitotic error before twinning and an unequal distribution of mosaicism in both embryos. So far the diagnosis of heterokaryotypic monozygotic pregnancy has always been made postnatally, with only one exception (Gonsoulin et al., 1990). In this case we suspected the presence of monozygotic twins ultrasonically because of the chorionic and amniotic membrane characteristics. Surprisingly the sex of the fetuses was discrepant. As one of them had hydrops and a structural heart defect, we carried out an amniocentesis, which revealed mosaicism [45,X/46,X,i(Y)(p10)] of both fetuses. The female fetus with a predominant 45,X set of chromosomes and the typical intrauterine signs of the Ullrich-Turner syndrome (massive hygroma colli, hydrops fetalis and multiple cardiac defects) died during the 25th week of gestation due to cardiac decompensation. The other fetus appeared to be male with a predominance of a 46,X,i(Y)(p10) set of chromosomes and was born a few days after the intrauterine death of the hydropic fetus. In conclusion, our observation shows that ultrasonic evidence of discordant fetal sex in twins does not necessarily exclude monozygosity.

Prenatal diagnosis in multiple pregnancy

Fetal abnormality is more common in multiple than in singleton pregnancies. This, together with the requirement to consider the risks with at least two babies to sample correctly each fetus and to undertake accurately-targeted selective termination, amounts to a major challenge for obstetricians involved in prenatal diagnosis. Early determination of chorionicity should be routine, since this influences not only the genetic risks but also the invasive procedure chosen for karyotyping or genotyping. Assessment of nuchal translucency identifies individual fetuses at risk of trisomy. Contrary to expectation, invasive procedures in twins appear to have procedure-related miscarriage rates that are similar to those in singletons. Instead, contamination remains a concern at chorionic villus sampling. Elective late karyotyping of fetuses may have a role in some countries. Whereas management options for discordant fetal abnormality are relatively straightforward in dichorionic pregnancies, monochorionic pregnancies are at risk of co-twin sequelae after any single intrauterine death. Techniques have now been developed to occlude completely the cord vasculature by laser and/or ultrasound guided bipolar diathermy. Given the complexities associated with prenatal diagnosis, all invasive procedures in multiple pregnancies should be performed in tertiary referral centres.

First trimester ultrasonography of multiple gestations: a review

The objective was to review current literature pertaining to first trimester ultrasonography of multiple gestations. To this goal, all manuscripts published in the English language regarding this topic were selected and reviewed in a MEDLINE search from 1966 through May 1998. Additional sources were identified through cross-referencing. Current widespread application of first trimester ultrasonography and especially transvaginal sonography has introduced a new dimension in both diagnostic and management aspects of multiple gestations. Application of first trimester ultrasonography in multiple gestations enables an earlier and more precise depiction of important anatomical details regarding fetal viability, chorionicity, pregnancy outcome, structural abnormalities, pathophysiology of developmental disorders (such as twin reverse arterial perfusion [TRAP] sequence), early sonographic signs associated with fetal aneuploidy (nuchal translucency and abnormal crown-rump length), and potential fetal growth discordancy. First trimester ultrasonography also assists in guiding operative procedures including: amniocentesis, chorionic villus sampling, and selective fetal reduction. Enhanced information obtained with high-resolution, first trimester transvaginal ultrasonography is rapidly becoming a standard for establishing critical information that will assist clinicians to stratify management of multiple gestations. Given the increasing incidence of multiple gestations because of various assisted reproductive technology modalities, it is important that obstetricians become aware of the potential advantages of first trimester ultrasonography in clinical management of multiple gestations.

Screening for fetal abnormalities in multiple pregnancies

Multiple gestations account for 1-2% of all pregnancies but contribute disproportionately to the incidence of both perinatal loss and fetal structural abnormalities. Ultrasound examination provides essential information about screening for, and the management of, such defects, including accurate determination of chorionicity, assessment of risk, invasive testing and selective termination if appropriate.

The consequences of monochorionic placentation

Monochorionic (MC) placentation occurs in two-thirds of monozygous twins and approximately 0.3% of all spontaneous conceptions. Vascular anastomoses within the placenta allow intertwin transfusion to occur, which is in most cases a normal event. However, imbalance in this flow may lead to the clinical sequelae observed, with acute, chronic or reverse intertwin transfusion. Acute transfusion describes the events following the death of one of an MC twin pair in utero, with an approximately 1 in 4 chance of demise or neurological damage in the co-twin. Chronic intertwin transfusion, otherwise termed feto-fetal transfusion syndrome, arises following the gradual flux of blood from one twin (the donor) to its sibling fetus (the recipient). The ensuing polyhydramnios places the pregnancy at risk of preterm amniorrhexis and/or labour, notwithstanding the specific sequelae seen in these fetuses following chronic in utero insult. Reverse transfusion describes the most bizarre form of intertwin transfusion: acardiac twinning. Retrograde perfusion of one of the twins with deoxygenated blood leads to the formation of a non-viable acardiac parasite and a 'pump twin' struggling to maintain the cardiac output required to perfuse both twins.