A reassessment of biochemical marker distributions in trisomy 21-affected and unaffected twin pregnancies in the first trimester

Prenatal Genetic Screening in Twin Pregnancy

Twin pregnancy presents unique considerations for aneuploidy screening. Pre-test counseling regarding benefits, alternatives, and options for aneuploidy screening should be provided to all patients carrying twin pregnancy. This article aims to review the options for aneuploidy screening in twin pregnancy including the potential benefits and limitations.

Risk of fetal loss after chorionic villus sampling in twin pregnancy derived from propensity score matching analysis

OBJECTIVE

To estimate the risk of fetal loss associated with chorionic villus sampling (CVS) in twin pregnancy, using propensity score analysis.

METHODS

This was a multicenter cohort study of women with twin pregnancy undergoing ultrasound examination at 11-13 weeks' gestation, performed in eight fetal medicine units in which the leadership were trained at the Harris Birthright Research Centre for Fetal Medicine in London, UK, and in which the protocols for screening, invasive testing and pregnancy management are similar. The risk of death of at least one fetus was compared between pregnancies that had and those that did not have CVS, after propensity score matching (1:1 ratio). This procedure created two comparable groups by balancing the maternal and pregnancy characteristics that lead to CVS being performed, similar to how randomization operates in a randomized clinical trial.

RESULTS

The study population of 8581 twin pregnancies included 445 that had CVS. Death of one or two fetuses at any stage during pregnancy occurred in 11.5% (51/445) of pregnancies in the CVS group and in 6.3% (515/8136) in the non-CVS group (P < 0.001). The propensity score algorithm matched 258 cases that had CVS with 258 non-CVS cases; there was at least one fetal loss in 29 (11.2%) cases in the CVS group and in 35 (13.6%) cases in the matched non-CVS group (odds ratio (OR), 0.81; 95% CI, 0.48-1.35; P = 0.415). However, there was a significant interaction between the risk of fetal loss after CVS and the background risk of fetal loss; when the background risk was higher, the risk of fetal loss after CVS decreased (OR, 0.46; 95% CI, 0.23-0.90), while, in pregnancies with a lower background risk of fetal loss, the risk of fetal loss after CVS increased (OR, 2.45; 95% CI, 0.95-7.13). The effects were statistically significantly different (P-value of the interaction = 0.005). For a pregnancy in which the background risk of fetal loss was about 6% (the same as in our non-CVS population), there was no change in the risk of fetal loss after CVS, but, when the background risk was more than 6%, the posterior risk was paradoxically reduced, and when the background risk was less than 6%, the posterior risk increased exponentially; for example, if the background risk of fetal loss was 2.0%, the relative risk was 2.8 and the posterior risk was 5.6%.

CONCLUSION

In twin pregnancy, after accounting for the risk factors that lead to both CVS and spontaneous fetal loss and confining the analysis to pregnancies at lower prior risk, CVS seems to increase the risk of fetal loss by about 3.5% above the patient's background risk. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

First-Trimester Screening for Gestational Diabetes Mellitus in Twin Pregnancies

We previously reported a logistic regression model for prediction of GDM from maternal characteristics and medical history in 75,161 singleton pregnancies. In this study of 1376 twin and 13,760 singleton pregnancies recruited at 11-13 weeks' gestation, we extend the model to include terms for twin pregnancies. We found the respective odds of GDM in dichorionic and monochorionic twin pregnancies to be 1.36 (95% CI: 1.02-1.81) and 2.78 (95% CI: 1.72-4.48) times higher than in singleton pregnancies. In both singleton and twin pregnancies, the risk for GDM increased with maternal age and weight and birth weight z-score of a baby in a previous pregnancy and is higher in women with a previous pregnancy complicated by GDM; in those with a first- or second-degree relative with diabetes mellitus; in women of Black, East Asian, and South Asian racial origin; and in pregnancies conceived through the use of ovulation-induction drugs. In singleton pregnancies, at 10% and 20% false-positive rate, the detection rate was 43% and 58%, respectively. In twin pregnancies, using risk cut-offs corresponding to 10% and 20% false-positive rates in singletons, the respective false-positive rates were 27% and 47%, and the detection rates were 63% and 81%.

Fetal loss after chorionic villus sampling in twin pregnancy

OBJECTIVE

To estimate the chorionic villus sampling (CVS)-related risk of fetal loss in twin pregnancy after adjustment for chorionicity, nuchal translucency thickness (NT), intertwin discordance in crown-rump length (CRL), maternal demographic characteristics and serum pregnancy-associated plasma protein-A (PAPP-A) and free β-human chorionic gonadotropin (β-hCG).

METHODS

This was a multicenter study from eight fetal medicine units in which the leadership were trained at the Harris Birthright Research Centre for Fetal Medicine in London, UK, and in which the protocols for screening, invasive testing and pregnancy management are similar. Data were obtained prospectively from women with twin pregnancy undergoing routine ultrasound examination at 11-13 weeks' gestation. Multivariable logistic regression analysis with backward stepwise elimination was used to examine whether CVS provided a significant independent contribution to the prediction of risk of fetal loss after adjusting for maternal and pregnancy characteristics, including maternal age, racial origin and weight, method of conception, smoking status, parity, chorionicity, intertwin discordance in CRL, fetal NT ≥ 95^th percentile and free β-hCG and PAPP-A multiples of the median. Similarly, within the CVS group, multivariable logistic regression analysis was used to investigate the effect of the number of intrauterine needle insertions and size of the needle on the risk of fetal loss.

RESULTS

The study population of 8581 twin pregnancies undergoing ultrasound examination at 11-13 weeks' gestation included 316 dichorionic and 129 monochorionic twins that had CVS. First, in twin pregnancies undergoing CVS, compared to those not undergoing CVS, there was a 2-fold increased risk of fetal loss at < 24 weeks' gestation and of loss at any stage in pregnancy. Second, the factors providing a significant independent contribution to the prediction of miscarriage or fetal loss in twin pregnancy were increased maternal weight, black racial origin, monochorionicity, and more so monoamnionicity, large intertwin discordance in CRL and increased fetal NT, and, in the case of fetal loss at any stage, there was also a contribution from assisted conception and low serum PAPP-A. Third, after adjustment for maternal and pregnancy characteristics, CVS did not provide a significant contribution to the risk of fetal loss. Fourth, in twin pregnancies that had CVS, there was no significant contribution to fetal loss from the number of intrauterine needle insertions or needle size.

CONCLUSION

The 2-fold increased risk of fetal loss following CVS in twin pregnancy can, to a great extent, be explained by maternal and pregnancy characteristics rather than the invasive procedure itself. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

Performance of first trimester screening for Trisomy 21 in twin pregnancies

OBJECTIVES

To assess the performance of the Danish first trimester screening program in twin pregnancies.

METHOD

Retrospective, nation-wide, cohort study with data collected from the Danish Fetal Medicine Database (DFMD) and The Danish Central Cytogenetic Registry (DCCR). The cohort included all women with twin pregnancies participating in the national first trimester screening program for Trisomy 21. Risk assessment was based on maternal age, nuchal translucency (NT) thickness and, if available, biochemical markers (β-hCG and PAPP-A).

RESULTS

8812 twin pregnancies including 42 pregnancies with Trisomy 21 had a risk assessment between 2009 and 2017. The detection rate (DR) for pregnancies with a risk assessment based on maternal age and NT only (missing data on biochemical markers, n = 4693) was 69.6% (95% CI: 50.8%-88.4%) for a 6.3% false positive rate (FPR) (95% CI: 5.6%-7.0%), whereas for pregnancies with a risk assessment based on all three parameters (n = 4119) the DR was 89.5% (95% CI: 76.7%-100.0%) for a 7.2% FPR (95% CI: 6.4%-8.0%).

CONCLUSION

The DR of Trisomy 21 in twin pregnancies, seems as high as for singleton pregnancies, when using optimal screening techniques, but the FPR is nearly twice as high.

Diagnosis of fetal defects in twin pregnancies at routine 11-13-week ultrasound examination

OBJECTIVES

To examine the performance of the routine 11-13-week scan in detecting fetal defects in twin pregnancies and to examine if, in pregnancies with a fetal defect, compared to those with normal fetuses, there is increased incidence of nuchal translucency thickness (NT) ≥ 95^th and ≥ 99^th percentiles or intertwin discordance in crown-rump length (CRL) ≥ 10% and ≥ 15%.

METHODS

This was a retrospective analysis of prospectively collected data in twin pregnancies undergoing routine ultrasound examination for fetal anatomy, according to standardized protocols, at 11-13 weeks' gestation between 2002 and 2019. Pregnancies with known chromosomal abnormality were excluded. The final diagnosis of fetal defect was based on the results of postnatal examination in cases of live birth and on the findings of the last ultrasound examination in cases of pregnancy termination, miscarriage or stillbirth. The performance of the 11-13-week scan in the detection of fetal defects was determined.

RESULTS

The study population of 6366 twin pregnancies with two live fetuses at 11-13 weeks' gestation included 4979 (78.2%) dichorionic (DC) and 1387 (21.8%) monochorionic (MC) twin pregnancies. The main findings were: first, the overall incidence of fetal defects was higher in MC than in DC twins (2.8% vs 1.3%); second, the proportion of defects diagnosed in the first trimester was higher in MC than in DC twins (52.6% vs 27.1%); third, the pattern of defects in relation to detectability at the 11-13-week scan (always detectable, sometimes detectable and never detectable) was similar to that reported previously in singleton pregnancies; fourth, always-detectable defects included acrania, alobar holoprosencephaly, encephalocele, pentalogy of Cantrell, exomphalos, body-stalk anomaly, twin reversed arterial perfusion sequence and conjoined twins; fifth, the incidence of fetal NT ≥ 95^th percentile was higher in those with than in those without a defect (16.5% vs 4.5% in DC twins and 19.2% vs 5.9% in MC twins) and this was also true for NT ≥ 99^th percentile (8.3% vs 1.0% in DC twins and 15.4% vs 2.0% in MC twins); and sixth, the incidence of CRL discordance ≥ 10% was higher in those with than in those without a defect (20.2% vs 7.9% in DC twins and 33.8% vs 9.3% in MC twins) and this was also true for CRL discordance ≥ 15% (10.1% vs 1.9% in DC twins and 28.2% vs 2.8% in MC twins).

CONCLUSIONS

First, fetal defects are more common in MC than in DC twin pregnancies. Second, first-trimester detection of fetal defects in DC twin pregnancies is similar to that in singleton pregnancies. Third, first-trimester detectability of defects in MC twins is higher than in DC twins. Fourth, in twin pregnancies with a fetal defect, there is higher intertwin discordance in CRL and incidence of increased NT, but the predictive performance of screening by these markers is poor. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd.

Increased nuchal translucency at 11-13 weeks' gestation and outcome in twin pregnancy

OBJECTIVE

To investigate the value of increased fetal nuchal translucency thickness (NT) at the 11-13-week scan in the prediction of adverse outcome in dichorionic (DC), monochorionic diamniotic (MCDA) and monochorionic monoamniotic (MCMA) twin pregnancies.

METHODS

This was a retrospective analysis of prospectively collected data on twin pregnancies undergoing routine ultrasound examination at 11-13 weeks' gestation between 2002 and 2019. In pregnancies with no major defects or chromosomal abnormalities, we examined the value of increased NT ≥ 95^th percentile in one or both fetuses in the prediction of, first, miscarriage or death of one or both fetuses at < 20 and < 24 weeks' gestation in DC, MCDA and MCMA twin pregnancies, second, death of one or both fetuses or neonates at ≥ 24 weeks in DC, MCDA and MCMA twin pregnancies, third, development of twin-twin transfusion syndrome (TTTS) or selective fetal growth restriction (sFGR) treated by endoscopic laser surgery at < 20 and ≥ 20 weeks' gestation in MCDA pregnancies, and, fourth, either fetal loss or laser surgery at < 20 weeks' gestation in MCDA pregnancies.

RESULTS

The study population of 6225 twin pregnancies included 4896 (78.7%) DC, 1274 (20.5%) MCDA and 55 (0.9%) MCMA pregnancies. The incidence of NT ≥ 95^th percentile in one or both fetuses in DC twin pregnancies was 8.3%; in MCDA twins the incidence was significantly higher (10.4%; P = 0.016), but in MCMA twins it was not significantly different (9.1%; P = 0.804) from that in DC twins. In DC twin pregnancies, the incidence of high NT was not significantly different between those with two survivors and those with adverse outcome. In MCMA twin pregnancies, the number of cases was too small for meaningful assessment of the relationship between high NT and adverse outcome. In MCDA twin pregnancies with at least one fetal death or need for endoscopic laser surgery at < 20 weeks' gestation, the incidence of NT ≥ 95^th percentile was significantly higher than in those with two survivors (23.5% vs 9.8%; P < 0.0001). Kaplan-Meier analysis in MCDA twin pregnancies showed that, in those with NT ≥ 95^th percentile, there was significantly lower survival at < 20 weeks' gestation than in those with NT < 95^th percentile (P = 0.001); this was not the case for survival at ≥ 20 weeks (P = 0.960). The performance of screening by fetal NT ≥ 95^th percentile for prediction of either fetal loss or need for endoscopic laser surgery at < 20 weeks' gestation was poor, with a detection rate of 23.5% at a false-positive rate of 8.9%, and the relative risk, in comparison to fetal NT < 95^th percentile, was 2.640 (95% CI, 1.854-3.758; P < 0.0001). In MCDA twin pregnancies, the overall rate of fetal loss or need for laser surgery at < 20 weeks' gestation was 10.7% but, in the subgroups with NT ≥ 95^th and NT ≥ 99^th percentiles, which constituted 10.4% and 3.3% of the total, the rates increased to 24.1% and 40.5%, respectively.

CONCLUSIONS

In MCDA twin pregnancies with no major fetal abnormalities, measurement of NT at the 11-13-week scan is a poor screening test for adverse pregnancy outcome. However, the finding in one or both fetuses of NT ≥ 95^th percentile, and more so ≥ 99^th percentile, is associated with a substantially increased risk of fetal loss or need for endoscopic laser surgery at < 20 weeks' gestation. The extent to which closer monitoring and earlier intervention in the high-risk group can reduce these complications remains to be determined. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd.

First-trimester screening for trisomies in pregnancies with vanishing twin

OBJECTIVES

To examine multiples of the median (MoM) values of serum free beta-human chorionic gonadotropin (β-hCG) and pregnancy-associated plasma protein-A (PAPP-A) in a large series of pregnancies with a vanishing twin, determine the association of these values with the interval between embryonic death and blood sampling, and develop a model that would allow incorporation of these metabolites in first-trimester combined screening for trisomy.

METHODS

This was a retrospective study comparing maternal serum free β-hCG and PAPP-A levels at 11-13 weeks' gestation in 528 dichorionic pregnancies with a vanishing twin, including 194 (36.7%) with an empty gestational sac and 334 (63.3%) with a dead embryo, with those in 5280 normal singleton pregnancies matched for method of conception and date of examination. In vanishing-twin pregnancies with a dead embryo, marker levels were examined in relation to the estimated time between embryonic death and maternal blood sampling.

RESULTS

First, in pregnancies with a vanishing twin, median free β-hCG MoM was not significantly different from that in normal singleton pregnancies (1.000; 95% CI, 0.985-1.016 vs 0.995; 95% CI, 0.948-1.044; P = 0.849). Second, PAPP-A MoM was higher in vanishing-twin pregnancies than in normal singleton pregnancies (1.000; 95% CI, 0.985-1.015), both in the group with an empty gestational sac (1.165; 95% CI, 1.080-1.256; P = 0.0001) and in that with a dead embryo (1.175; 95% CI, 1.105-1.249; P < 0.0001). Third, in vanishing-twin pregnancies with a dead embryo, PAPP-A MoM was related inversely to the interval between estimated gestational age at embryonic demise and blood sampling (P < 0.0001). Fourth, in first-trimester screening for trisomy 21 in singleton pregnancies, the estimated detection rate, at a 5% false-positive rate, was 82% in screening by a combination of maternal age and fetal nuchal translucency thickness, and this increased to 86% with the addition of serum free β-hCG and to 91% with the addition of serum PAPP-A. Fifth, similar performance of screening can be achieved in pregnancies with a vanishing twin, provided the appropriate adjustments are made to the level of PAPP-A for the interval between estimated gestational age at embryonic demise and blood sampling.

CONCLUSIONS

First-trimester screening for trisomy in pregnancies with a vanishing twin should rely on a combination of maternal age, fetal nuchal translucency thickness and serum free β-hCG, as in singleton pregnancy, without the use of serum PAPP-A. Alternatively, PAPP-A can be included but only after appropriate adjustment for the interval between estimated gestational age at fetal demise and blood sampling. Copyright © 2019 ISUOG. Published by John Wiley & Sons Ltd.

Prenatal screening for Down syndrome in twin pregnancies: Estimates of screening performance based on 61 affected and 7302 unaffected twin pregnancies

The aims of this study were to determine whether assumptions used in prenatal screening for Down syndrome in twin pregnancies are valid and derive estimates of risk and screening performance in twin pregnancies using observed data. Data were collected on nuchal translucency, chorionicity, pregnancy associated plasma protein-A (PAPP-A), and free ß human chorionic gonadotrophin (free ß-hCG) from 61 twin pregnancies with Down syndrome and 7302 unaffected twin pregnancies. Distribution parameters were determined and used to estimate screening performance. The assumption that proportional differences in serum marker levels in affected and unaffected singleton pregnancies apply to twin pregnancies was not confirmed. Median free β-hCG value in monochorionic affected twin pregnancies (2.63 multiples of the median [MoM]; 95% CI, 1.79-3.22 MoM) was lower than that assuming proportionality (3.76 MoM), and the median PAPP-A value in dichorionic affected twin pregnancies (1.88 MoM; 95% CI, 1.60-2.17 MoM) was higher than that based on proportionality (1.33 MoM). The detection rate was 87% for a 3% false-positive rate in monochorionic twin pregnancies and 74% in dichorionic twin pregnancies compared with 86% in singleton pregnancies. Estimates of screening performance in Down syndrome twin pregnancies do not need to rely on assumptions and can take account of chorionicity and gestational age.

Validation of a Single-Nucleotide Polymorphism-Based Non-Invasive Prenatal Test in Twin Gestations: Determination of Zygosity, Individual Fetal Sex, and Fetal Aneuploidy

We analyzed maternal plasma cell-free DNA samples from twin pregnancies in a prospective blinded study to validate a single-nucleotide polymorphism (SNP)-based non-invasive prenatal test (NIPT) for zygosity, fetal sex, and aneuploidy. Zygosity was evaluated by looking for either one or two fetal genome complements, fetal sex was evaluated by evaluating Y-chromosome loci, and aneuploidy was assessed through SNP ratios. Zygosity was correctly predicted in 100% of cases (93/93; 95% confidence interval (CI) 96.1%-100%). Individual fetal sex for both twins was also called with 100% accuracy (102/102; 95% weighted CI 95.2%-100%). All cases with copy number truth were also correctly identified. The dizygotic aneuploidy sensitivity was 100% (10/10; 95% CI 69.2%-100%), and overall specificity was 100% (96/96; 95% weighted CI, 94.8%-100%). The mean fetal fraction (FF) of monozygotic twins (n = 43) was 13.0% (standard deviation (SD), 4.5%); for dizygotic twins (n = 79), the mean lower FF was 6.5% (SD, 3.1%) and the mean higher FF was 8.1% (SD, 3.5%). We conclude SNP-based NIPT for zygosity is of value when chorionicity is uncertain or anomalies are identified. Zygosity, fetal sex, and aneuploidy are complementary evaluations that can be carried out on the same specimen as early as 9 weeks' gestation.

Routine first-trimester screening for fetal trisomies in twin pregnancy: cell-free DNA test contingent on results from combined test

OBJECTIVE

To report on the routine clinical implementation of cell-free DNA (cfDNA) analysis of maternal blood for trisomies 21, 18 and 13, contingent on the results of the first-trimester combined test in twin pregnancy.

METHODS

Screening for trisomies 21, 18 and 13 was carried out in 959 twin pregnancies by assessment of a combination of maternal age, fetal nuchal translucency thickness, and serum free β-human chorionic gonadotropin and pregnancy-associated plasma protein-A at 11-13 weeks' gestation in two UK NHS hospitals. Women in the high-risk group (risk ≥ 1 in 100) were offered the option of invasive testing, cfDNA testing or no further testing, and those in the intermediate-risk group (risk 1 in 101 to 1 in 2500 in the first phase of the study and 1 in 101 to 1 in 500 in the second phase) were offered cfDNA or no further testing. The trisomic status of the pregnancies was determined by prenatal or postnatal karyotyping or examination of the neonates.

RESULTS

In 42 (4.4%) of the 959 pregnancies, there was termination, miscarriage or stillbirth with no known karyotype or there was loss to follow-up. The 917 pregnancies with known trisomic status of both twins included six that were discordant for trisomy 21, four that were discordant for trisomy 18 and 907 with no trisomy 21, 18 or 13. Following combined screening, 47 (5.1%), 203 (22.1%) and 667 (72.7%) of the pregnancies were classified as high risk, intermediate risk and low risk, respectively. The high-risk group included five (83.3%) cases of trisomy 21 and three (75.0%) of trisomy 18. The cfDNA test was carried out in 224 pregnancies and results were provided in 214 (95.5%); this group included six pregnancies with trisomy 21, three with trisomy 18 and 206 with no trisomy 21, 18 or 13. The cfDNA test classified correctly as screen positive all six cases of trisomy 21 and two of the three with trisomy 18, and as screen negative for each of the trisomies all 206 unaffected pregnancies. Contingent screening led to prenatal detection of all cases of trisomy 21 and three of four with trisomy 18.

CONCLUSION

This study has demonstrated the feasibility of introducing cfDNA testing, contingent on the results of the first-trimester combined test for major trisomies, in a routine population of twin pregnancies. Copyright © 2018 ISUOG. Published by John Wiley & Sons Ltd.

Screening for Common Fetal Trisomies in Twin Pregnancies: First-Trimester Combined, Cell-Free DNA, or Both?

OBJECTIVE

To examine the distribution of risks for fetal trisomies after first-trimester combined screening in twins and to investigate different strategies for clinical implementation of cell-free DNA (cfDNA) testing.

METHODS

We retrospectively analyzed all twin pregnancies undergoing first-trimester combined screening over a 10 years' period. The population was stratified according to various risk cut-offs, and we examined different screening strategies for implementation of cfDNA testing in terms of impact on invasive testing rate, cfDNA test failure rate, and economic costs.

RESULTS

We included 572 twin pregnancies: 480 (83.92%) dichorionic and 92 (16.08%) monochorionic. Performing a first-line combined screening and offering cfDNA testing to the group with a risk between 1 in 10 and 1 in 1,000, would lead to an invasive testing rate of 2.45%, and cfDNA testing would be performed in 22.20% of the population. This strategy would be cost-neutral compared to universal combined screening alone.

CONCLUSIONS

First-trimester combined screening results can be used to stratify twin pregnancies into different risk categories and select those that could be offered cfDNA testing. A contingent screening strategy would substantially decrease the need for invasive testing in twins and it would be cost-neutral compared to combined testing alone.

Cell-free fetal DNA analysis in maternal plasma as screening test for trisomies 21, 18 and 13 in twin pregnancy

OBJECTIVES

To evaluate in twin pregnancy the utility of non-invasive prenatal testing using circulating cell-free fetal DNA (cfDNA) in screening for the three main autosomal fetal trisomies.

METHODS

cfDNA testing was offered to 492 patients with a twin pregnancy without ultrasound anomaly as a first-line screening test or after routine serum screening. Data were collected prospectively and a retrospective analysis was performed. cfDNA analysis was performed by massively parallel sequencing. The fetal-fraction threshold used for test evaluation was 8%. Regression analysis was performed to investigate the effect on the test failure rate of maternal and pregnancy characteristics, and the performance of the test was also reported.

RESULTS

cfDNA analysis was performed as a first-line test (after the first-trimester scan) in 377 patients and following serum screening in 115. Of the 420 pregnancies for which outcome was available and cfDNA screening was assessed, 78.7% were dichorionic-diamniotic. The test failed on the first attempt in 12 (2.9%) pregnancies, and regression analysis demonstrated that only maternal weight was a significant independent predictor of test failure. A result was subsequently achieved in the 10 cases for which a second sample was obtained. cfDNA analysis identified all three cases of trisomy 21 and the only case of trisomy 18. For trisomy 21, the specificity was 99.8% (95% CI, 98.7-100.0%). When considering pregnancies according to whether they were conceived spontaneously or after assisted reproductive technology, there were no significant differences in terms of maternal weight or no-result rate for cfDNA screening between these two groups.

CONCLUSIONS

In twin pregnancy without fetal ultrasound abnormality, cfDNA screening for trisomies 21, 18 and 13 had a high success rate and good performance. Therefore, in routine practice, cfDNA analysis could be considered as a first- or second-line screening test. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

[Cell-free fetal DNA analysis in maternal plasma as a screening test for trisomy 21 in twin pregnancies]

OBJECTIVES

To evaluate the performance of noninvasive prenatal testing by cell-free circulating fetal DNA in maternal blood (cfDNA) in screening for trisomies 21 in twin pregnancies.

METHODS

CfDNA was performed in 492 patients with twin pregnancies without ultrasound anomalies in the first trimester as a first-line screening test or after serum screening. Data were collected prospectively and a retrospective analysis was done. CfDNA was executed by massive parallel technique. The fetal fraction threshold for test evaluation was 8%. Regression analysis was performed to evaluate the effect of different parameters on the test failure rate. Performance of the test was also considered.

RESULTS

In 377 patients, the test was prescribed first line and in 115 after standard serum screening. Twelve tests (2.9%) have initially failed on the 420 pregnancies with available outcomes and regression analysis found only maternal weight as a significant independent factor of test failure. A second test was performed on 10 patients, all of them had an available result. cfDNA identified all 3 cases of trisomy 21. The sensitivity was 100.0% (95% CI [29.2-100.0%]) and specificity was 99.8% (95% CI [98.7-100.0%]). There was no significant difference between spontaneous pregnancies and those induced by assisted reproductive technologies (ART), in terms of fetal fraction percentage, no-call results for cfDNA screening, maternal weight, or test performance between the two groups.

CONCLUSION

In twin pregnancies without fetal ultrasound abnormalities, the performance and success rate of the cfDNA are excellent. Therefore, cfDNA could be offered in routine practice as a first-line screening test in this population.

Competing-risks model in screening for pre-eclampsia in twin pregnancy according to maternal factors and biomarkers at 11-13 weeks' gestation

OBJECTIVE

To develop a model for screening for pre-eclampsia (PE) in twin pregnancies based on maternal demographic characteristics and medical history and biomarkers at 11-13 weeks' gestation.

METHODS

This was a screening study in twin pregnancies at 11-13 weeks' gestation. Bayes theorem was used to combine the a-priori risk from maternal factors with various combinations of uterine artery pulsatility index (UtA-PI), mean arterial pressure (MAP), serum pregnancy-associated plasma protein-A (PAPP-A) and placental growth factor (PlGF) multiples of the median (MoM) values. The performance of screening for PE requiring delivery at < 32, < 37 and < 42 weeks' gestation was estimated in 1100 twin pregnancies and 35 948 singleton pregnancies with complete data on UtA-PI, MAP, PlGF and PAPP-A.

RESULTS

In twin pregnancies that developed PE, the values of MAP and UtA-PI were increased and the values of PlGF and PAPP-A were decreased. The distributions of log₁₀ MoM values of biomarkers with gestational age at delivery were similar to those that were previously reported in singleton pregnancies and it was therefore assumed that the same model could be used for both singleton and twin pregnancies. The performance of screening for PE by maternal factors was improved by the addition of MAP, UtA-PI and PlGF; there was no further improvement with the addition of PAPP-A. In a mixed population of singleton and twin pregnancies, combined screening by maternal factors, MAP, UtA-PI and PlGF and risk cut-off of 1 in 75 for PE at < 37 weeks, the detection rate of PE at < 32, < 37 and < 42 weeks in singleton pregnancies was 91%, 77% and 57%, respectively, at a screen-positive rate (SPR) of 13%; the respective rates for twin pregnancies were 100%, 99% and 97%, at a SPR of 75%.

CONCLUSION

First-trimester combined screening for PE in singleton pregnancies can be adapted for screening in twins, leading to detection of nearly all affected cases but at a high SPR. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

Prospective first-trimester screening for trisomies by cell-free DNA testing of maternal blood in twin pregnancy

OBJECTIVES

First, to examine in twin pregnancies the performance of first-trimester screening for fetal trisomies 21, 18 and 13 by cell-free (cf) DNA testing of maternal blood and, second, to compare twin and singleton pregnancies regarding the distribution of fetal fraction of cfDNA and rate of failure to obtain a result.

METHODS

This was a prospective study in 438 twin and 10 698 singleton pregnancies undergoing screening for fetal trisomies by cfDNA testing at 10 + 0 to 13 + 6 weeks' gestation. Chromosome-selective sequencing of cfDNA was used and, in twin pregnancies, an algorithm was applied that relies on the lower fetal fraction contributed by the two fetuses. Multivariate regression analysis was used to determine significant predictors of fetal fraction and a failed result.

RESULTS

In twin pregnancies, the median fetal fraction was lower (8.0% (interquartile range (IQR), 6.0-10.4%) vs 11.0% (IQR, 8.3-14.4%); P < 0.0001) and failure rate after first sampling was higher (9.4% vs 2.9%; P < 0.0001) compared to in singletons. Multivariate logistic regression analysis demonstrated that the risk of test failure increased with increasing maternal age and body mass index and decreased with fetal crown-rump length. The risk was increased in women of South Asian racial origin and in pregnancies conceived by in-vitro fertilization (IVF). The main contributor to the higher rate of failure in twins was conception by IVF which was observed in 9.5% of singletons and 56.2% of twins. In the 417 twin pregnancies with a cfDNA result after first or second sampling, the detection rate was 100% (8/8) for trisomy 21 and 60% (3/5) for trisomies 18 or 13, at a false-positive rate (FPR) of 0.25% (1/404). In the 10 530 singleton pregnancies with a cfDNA result after first or second sampling, the detection rate was 98.7% (156/158) for trisomy 21 and 80.3% (49/61) for trisomies 18 or 13, at a FPR of 0.22% (23/10 311).

CONCLUSIONS

In twin pregnancies undergoing first-trimester screening for trisomies by cfDNA testing, the fetal fraction is lower and failure rate higher compared to in singletons. The number of trisomic twin pregnancies examined was too small for an accurate assessment of performance of screening, but it may be similar to that in singleton pregnancies. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd.

First-trimester Ultrasound and Aneuploidy Screening in Multifetal Pregnancies

First-trimester ultrasound provides valuable information to help optimize the management of multifetal pregnancies. First trimester ultrasound the use of ultrasound and screening for aneuploidy has been well studied in singleton pregnancies. While evidence supporting the use of ultrasound in multiple gestations is well established, aneuploidy screening continues to evolve and its role in the prenatal setting has been less well studied. We review the importance of early first trimester ultrasound in assessing gestational age and chorionicity and early identification of anomalies, and review the various methods and limitations of aneuploidy screening and invasive diagnostic procedures in multiples.

Ten Years of Experience with First-Trimester Screening for Fetal Aneuploidy Employing Biochemistry from Gestational Weeks 6+0 to 13+6

Objectives: To validate the performance of first-trimester screening for fetal aneuploidy employing blood samples drawn in gestational weeks 6-13. Methods: Prospective combined first-trimester screening for fetal aneuploidy in Denmark was validated in two large datasets: (1) a dataset from the Central Denmark Region including 147,768 pregnancies from October 2003 to October 2013, and (2) a national dataset including 220,739 pregnancies from January 2008 to August 2011. Results: For trisomy 21, the weekly median multiple of the median (MoM) increased from 0.37 in week 6 to 0.70 in week 13 (pregnancy-associated plasma protein-A), and from 0.99 in week 6 to 2.68 in week 13 (free βhCG). The overall detection rate (DR) for fetal trisomy 21 was 91.2%. Employing blood samples from gestational week 9, the DR was 97% (p = 0.05). For fetal trisomy 18, trisomy 13 and triploidy, the overall DRs after first-trimester screening were 79.5, 86 and 85%. In the national dataset, the overall DR for trisomy 21 was 86.3% ranging from 89 (weeks 9 and 10) to 80% (weeks 12 and 13). Conclusion: The results from both datasets show that blood sampling in gestational weeks 9-10 is a robust and high-performance strategy, which can be applied for routine first-trimester screening in clinical practice.

Screening and Invasive Testing in Twins

Prenatal screening and testing for trisomy 21 in twin pregnancies poses a number of challenges: the exact estimate of the a priori risk of trisomy 21, the choice of prenatal screening test and/or invasive techniques to employ for the diagnosis and the impact of the result on the options of treatment in case of discordant results within a twin pair or among multiples. These different aspects are discussed below while recognizing that many issues remain unresolved.

Nuchal translucency in twins according to mode of assisted conception and chorionicity

OBJECTIVES

The impact of nuchal translucency (NT) screening in twins conceived after use of assisted reproduction technology (ART) has not been addressed properly in the literature. The aim of the current study was to assess, in a large cohort from a single center, NT in twins following various modes of ART as compared with NT in spontaneously-conceived twins and to differentiate results with regard to chorionicity.

METHODS

Retrospective data from 825 twin pregnancies were available for this study. All pregnant women underwent NT screening and chorionicity determination between 11 + 0 and 13 + 6 weeks' gestation in our center. Information about mode of conception and pregnancy outcome was obtained from hospital records and by telephone interviews. Twins were classified as having been conceived after in-vitro fertilization (IVF) with the woman's own or a donated embryo, after non-IVF ART or spontaneously. NT distribution in subgroups of fetuses was compared on the basis of the multiple of the normal median (MoM), SD of log10 MoM estimated by the 90(th) -10(th) centile range divided by 2.563 and the proportion of NT values exceeding the normal 95(th) centile which was 1.641 MoM in the singletons.

RESULTS

NT values were statistically significantly higher in ART pregnancies as compared with spontaneously-conceived pregnancies (Wilcoxon rank-sum test, P < 0.05) with medians of 1.073 and 1.038 MoM, respectively, but the proportion with values above the normal 95(th) percentile did not differ (chi-square test, P = 0.89). Among the ART twins, NT values were significantly higher for those conceived using IVF methods (P < 0.005), with a median of 1.082 MoM, compared with a median of 1.022 MoM using other methods. However, although a greater proportion exceeded the 95(th) centile (7.3% compared with 4.8%) this difference did not reach statistical significance (P = 0.17). There were no differences in NT values according to chorionicity (Wilcoxon rank-sum test, P = 0.75). The standard deviation of log10 NT was similar for all subgroups of twins. The correlation coefficient between fetuses in ART pregnancies was statistically significantly lower than that in spontaneously-conceived pregnancies (P < 0.05) but no significant differences were found between types of ART or according to chorionicity.

CONCLUSIONS

There was a small but significant increase in NT levels among ART pregnancies, which appeared to be confined to those conceived using IVF, and a small reduction in the correlation between fetuses. These effects probably have little impact on the estimated risk based on algorithms in use today.

Down syndrome screening in assisted conception twins: an iatrogenic medical challenge

OBJECTIVE

The objective of this study was to provide a critical analysis of the impact of assisted conception on prenatal screening for Down syndrome (DS) in twin pregnancies and the value of various screening modalities for early detection of anomalies.

METHODS

The literature was searched using PubMed and the Cochrane Library focusing on prenatal screening and antenatal care of assisted-conception twin pregnancies.

RESULTS

Serum screening alone is of limited value in detecting aneuploid twins, because the unaffected cotwin can "mask" the abnormal serum results of an affected one. In addition, this test can designate the pregnancy as at high risk but not identify the affected fetus. Nuchal translucency (NT) screening is the best available modality and a highly effective screening method for twin pregnancies. Among twins, NT alone has a 69% DS detection rate, first-trimester combined NT and serum biochemistry has a 72% DS detection rate, and an integrated screen will have an 80% DS detection rate at a 5% FPR. The data in the literature concerning the effect of assisted conception on maternal serum screening markers in twin pregnancies are scarce.

CONCLUSIONS

Down syndrome screening in assisted-conception twins presents clinical and technical challenges. Therefore, assisted-conception twins need close monitoring from conception to delivery, by a practitioner familiar with the available screening modalities and their relative accuracy.

Prevalence and risk of Down syndrome in monozygotic and dizygotic multiple pregnancies in Europe: implications for prenatal screening

OBJECTIVE

To determine risk of Down syndrome (DS) in multiple relative to singleton pregnancies, and compare prenatal diagnosis rates and pregnancy outcome.

DESIGN

Population-based prevalence study based on EUROCAT congenital anomaly registries.

SETTING

Eight European countries.

POPULATION

14.8 million births 1990-2009; 2.89% multiple births.

METHODS

DS cases included livebirths, fetal deaths from 20 weeks, and terminations of pregnancy for fetal anomaly (TOPFA). Zygosity is inferred from like/unlike sex for birth denominators, and from concordance for DS cases.

MAIN OUTCOME MEASURES

Relative risk (RR) of DS per fetus/baby from multiple versus singleton pregnancies and per pregnancy in monozygotic/dizygotic versus singleton pregnancies. Proportion of prenatally diagnosed and pregnancy outcome.

STATISTICAL ANALYSIS

Poisson and logistic regression stratified for maternal age, country and time.

RESULTS

Overall, the adjusted (adj) RR of DS for fetus/babies from multiple versus singleton pregnancies was 0.58 (95% CI 0.53-0.62), similar for all maternal ages except for mothers over 44, for whom it was considerably lower. In 8.7% of twin pairs affected by DS, both co-twins were diagnosed with the condition. The adjRR of DS for monozygotic versus singleton pregnancies was 0.34 (95% CI 0.25-0.44) and for dizygotic versus singleton pregnancies 1.34 (95% CI 1.23-1.46). DS fetuses from multiple births were less likely to be prenatally diagnosed than singletons (adjOR 0.62 [95% CI 0.50-0.78]) and following diagnosis less likely to be TOPFA (adjOR 0.40 [95% CI 0.27-0.59]).

CONCLUSIONS

The risk of DS per fetus/baby is lower in multiple than singleton pregnancies. These estimates can be used for genetic counselling and prenatal screening.

Cell-free DNA analysis for trisomy risk assessment in first-trimester twin pregnancies

OBJECTIVE

To examine the clinical implementation of chromosome-selective sequencing of cell-free DNA (cfDNA) in maternal blood and an algorithm that relies on the lower fetal fraction contribution of the 2 fetuses in the assessment of risk for trisomies in twin pregnancies.

METHODS

Risk for trisomies 21, 18 and 13 by cfDNA testing were estimated in stored plasma samples obtained at 11-13 weeks' gestation from 207 pregnancies with known outcome and prospectively in 68 twin pregnancies undergoing screening at 10-13 weeks.

RESULTS

Risk scores for trisomies were provided for 192 (92.8%) of stored plasma and for 63 (92.6%) of the prospective cases. In the retrospective study, 10 of 11 trisomic pregnancies were correctly identified with no false positive results. In the prospective study, 3 trisomic pregnancies were correctly identified with no false positive results. The median of the lower fetal fraction in the prospective study of twins was 7.4% (IQR range 5.9-10.0%), which was lower than in our previous study in singletons (median 10.0%, IQR 7.8-13.0%).

CONCLUSIONS

cfDNA testing in twins is feasible but the reporting rate of results is lower than in singletons due to a lower fetal fraction.

First trimester maternal serum placental growth factor levels in twin pregnancies

BACKGROUND

An understanding of the normal behavior of biochemical markers in twin pregnancies is necessary in order to offer prenatal screening to this subgroup. This study investigates the levels of first trimester maternal serum placental growth factor (PlGF) in twin and singleton pregnancies.

METHODS

The PlGF concentrations were measured by an automated assay in the first trimester maternal serum of 440 dichorionic twin, 116 monochorionic twin, and 607 singleton pregnancy samples thawed from frozen storage.

RESULTS

The PlGF concentrations in singleton levels were predicted by gestational age, maternal ethnicity, and smoking status. Following the correction for these variables, PlGF levels were, on average, 41% higher in dichorionics, but only 16% higher in monochorionics, compared to singleton pregnancies.

CONCLUSIONS

First trimester maternal serum PlGF levels are increased in twin pregnancies compared with singleton pregnancies, but to less of an extent than is common with other screening markers, especially in monochorionic twins.

Ultrasound in twins: dichorionic and monochorionic

The objective of this review is to assess the evidence that supports the use of ultrasound in twin pregnancies. Although many of the indications for obstetric ultrasound are the same in both singleton and multiple gestations, there are special considerations as well as unique conditions in twins that require additional imaging studies. The reasons for ultrasound in twins include pregnancy dating, determination of chorionicity, nuchal translucency assessment, anatomical survey, placental evaluation, cervical length assessment, routine fetal growth, and serial surveillance of pregnancies complicated by anomalies, cervical shortening, fetal growth disturbances, and amniotic fluid abnormalities. Twins with monochorionic placentation require heightened scrutiny for monoamnionicity, conjoined twins, twin reversed arterial perfusion (TRAP) syndrome, twin-twin transfusion syndrome, unequal placental sharing with discordant twin growth or selective intrauterine fetal growth restriction (IUGR), twin anemia-polycythemia sequence (TAPS), and single fetal demise. Ultrasound is essential for the detection and management of conditions that can complicate dichorionic and monochorionic twin pregnancies.

Maternal thyroid function at gestational weeks 11-13 in twin pregnancies

BACKGROUND

Thyroid disease during pregnancy may be associated with increased risk of various pregnancy complications. It is known that serum thyrotropin (TSH) is suppressed because of the increased hormone production induced by human chorionic gonadotrophin (hCG) in early pregnancy, and that higher hCG levels in twin pregnancies may cause a more pronounced physiologic suppression. The recognition of this phenomenon is important in order to avoid unnecessary concerns and to correctly establish the diagnosis of overt thyroid disease in twin pregnancies. The aim of this study was to establish reference ranges of maternal serum TSH and free thyroxine (FT4) at gestational weeks 11-13 in twin pregnancies.

METHODS

This is a case series of 177 dichorionic and 58 monochorionic twin pregnancies with normal outcomes, and 19 monochorionic pregnancies complicated by severe twin-twin transfusion syndrome. Maternal serum concentrations of TSH, FT4, antithyroperoxidase, and antithyroglobulin antibodies were measured at gestational weeks 11-13. The measured TSH and FT4 were converted to multiple of median (MoM) of normal singleton pregnancies and MoM values in the different groups were compared.

RESULTS

In the antibody-negative twin pregnancies with normal outcomes, compared to singletons, serum TSH MoM was lower (median 0.62 [interquartile range [IQR 0.16-1.18] vs. 1.01 [IQR 0.61-1.51]; p < 0.0001), FT4 MoM was not significantly different (median 0.98 [IQR 0.91-1.08] vs. 0.99 [IQR 0.91-1.09]; p = 0.975), and free β-hCG MoM was higher (median 1.91 [IQR 1.33-2.59] vs. 0.98 [IQR 0.66-1.50]; p < 0.0001). In the antibody-positive group (n = 37), compared to the negative group (n = 198), the median TSH was higher, but FT4 and free β-hCG were not significantly different. In the twin-twin transfusion syndrome group, compared to normal twin pregnancies, TSH, FT4, and free β-hCG were not significantly different.

CONCLUSION

In twins, compared to singleton pregnancies, TSH is lower but FT4 is not significantly different. These reference ranges of thyroid hormones in twins can form the basis for the study of early thyroid function in pathological pregnancies and the investigation of the consequences of overt and subclinical hypothyroidism on twin pregnancy outcome.

Obstetrical complications associated with first-trimester screening markers in twin pregnancies

AIM

The aim of this retrospective study was to investigate the relation between perinatal complications in twin pregnancies and the combination of first-trimester maternal serum pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotrophin (β-hCG) levels.

MATERIAL AND METHODS

The study was conducted at the Perinatology Department of Baskent University between January 2005 and December 2011. In total, 104 patients with twin pregnancies were recruited. The association of first-trimester maternal serum-screening markers with perinatal complications was investigated for small-for-gestational-age (SGA) fetuses or intrauterine growth restriction (IUGR), preterm delivery (before 34 weeks of gestation), pregnancy-induced hypertension (PIH), and growth discordance between fetuses.

RESULTS

Low serum PAPP-A (below 10th percentile) levels were not correlated with preterm labor, PIH, or IUGR in twin gestations. Elevated free β-hCG levels (>90th percentile) were linked to increased rates of PIH and small-for-gestational-age fetuses or IUGR.

CONCLUSION

Elevated first-trimester free β-hCG was related to adverse pregnancy outcomes in twin pregnancies, whereas low PAPP-A levels were not linked to adverse pregnancy outcomes.

Analysis of three different strategies in prenatal screening for Down's syndrome in twin pregnancies

OBJECTIVES

To compare the performance of three different strategies in prenatal screening for Down's syndrome in twins [nuchal translucency, the combined test, the combined test + ductus venosus pulsatility index (DVPI)].

METHODS

We included 277 twin pregnancies with two cases of trisomy 21 (both dichorionic). We performed a computer simulation of Down's syndrome NT screening, combined test screening and the combined test with the addition of DVPI screening using the commercialized software SsdwLab6. The strategies were compared using the area under the receiver operating characteristic curve.

RESULTS

NT screening false-positive rate (FPR) was 10.9% (95% CI: 8.3-13.5). The combined test FPR was 6.2% (95% CI: 4.1-8.2%) and the combined test plus DVPI was 6% (95% CI: 4-8). FPR was higher in advanced maternal age patients. Detection rate was 100% in all cases. The area under the curve was 0.987 (95% CI: 0.972-0.994) in NT screening; 0.987 (95% CI: 0.978-0.997) in the combined test and 0.983 (95% CI: 0.977-0.996) in the combined test + DVPI.

CONCLUSIONS

Down's syndrome screening is feasible in twins with low FPR. The results of this study are similar to the results achieved in singletons. The combined test appears to be the most effective. The addition of DVIP does not significantly improve the prenatal screening for trisomy 21.

Trends in the prevalence, risk and pregnancy outcome of multiple births with congenital anomaly: a registry-based study in 14 European countries 1984-2007

OBJECTIVE

To assess the public health consequences of the rise in multiple births with respect to congenital anomalies.

DESIGN

Descriptive epidemiological analysis of data from population-based congenital anomaly registries.

SETTING

Fourteen European countries.

POPULATION

A total of 5.4 million births 1984-2007, of which 3% were multiple births.

METHODS

Cases of congenital anomaly included live births, fetal deaths from 20 weeks of gestation and terminations of pregnancy for fetal anomaly.

MAIN OUTCOME MEASURES

Prevalence rates per 10,000 births and relative risk of congenital anomaly in multiple versus singleton births (1984-2007); proportion prenatally diagnosed, proportion by pregnancy outcome (2000-07). Proportion of pairs where both co-twins were cases.

RESULTS

Prevalence of congenital anomalies from multiple births increased from 5.9 (1984-87) to 10.7 per 10,000 births (2004-07). Relative risk of nonchromosomal anomaly in multiple births was 1.35 (95% CI 1.31-1.39), increasing over time, and of chromosomal anomalies was 0.72 (95% CI 0.65-0.80), decreasing over time. In 11.4% of affected twin pairs both babies had congenital anomalies (2000-07). The prenatal diagnosis rate was similar for multiple and singleton pregnancies. Cases from multiple pregnancies were less likely to be terminations of pregnancy for fetal anomaly, odds ratio 0.41 (95% CI 0.35-0.48) and more likely to be stillbirths and neonatal deaths.

CONCLUSIONS

The increase in babies who are both from a multiple pregnancy and affected by a congenital anomaly has implications for prenatal and postnatal service provision. The contribution of assisted reproductive technologies to the increase in risk needs further research. The deficit of chromosomal anomalies among multiple births has relevance for prenatal risk counselling.