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

Objective: To examine the performance of the routine 11-13 weeks scan in detecting fetal defects in twin pregnancies and to examine if in pregnancies with fetal defects, compared to those with normal fetuses, there is increased incidence of nuchal translucency (NT) thickness ≥95th and ≥99th 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 abnormalities were excluded. The final diagnosis of fetal defects was based on the results of postnatal examination in the case of livebirths and on the findings of the last ultrasound examination in the cases of pregnancy termination, miscarriage or stillbirth. The performance of the 11-13 weeks scan in the detection of fetal defects was determined. Results: The study population of 6,366 twin pregnancies with two live fetuses at 11-13 weeks’ gestation, included 4,979 (78.2%) dichorionic (DC) and 1,387 (21.8%) monochorionic (MC) twin pregnancies. The main findings were: first, the overall incidence of fetal defects was higher in MC than 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 weeks scan, always detectable, sometimes detectable and never detectable, was similar to that previously reported in singleton pregnancies; fourth, always detectable defects included acrania, alobar holoprosencephaly, encephalocele, pentalogy of Cantrell, exomphalos, body stalk anomaly, TRAP sequence and conjoined twins; fifth, the incidence of fetal NT ≥95th percentile was higher in those with than without defects (16.5% vs. 4.5% in DC twins and 19.2% vs. 5.9% in MC twins) and this was also true for NT >99th 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 without defects (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, detectability of defects in MC twins is higher than in DC twins, fourth, in twin pregnancies with fetal defects there is a higher intertwin discordance in CRL and incidence of high NT, but the predictive performance of screening by these markers is poor