Diagnosis of fetal defects in twin pregnancies at routine 11
A study was done 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 an increased incidence of nuchal translucency thickness or intertwin discordance in crown-rump length .

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 a fetal defect was based on the results of the 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.
The study population of 6366 twin pregnancies with two live fetuses at 11–13 weeks' gestation included 4979 dichorionic (DC) and 1387 monochorionic (MC) twin pregnancies. The main findings were: first, the overall incidence of fetal defects was higher in MC than in DC twins second, the proportion of defects diagnosed in the first trimester was higher in MC than in DC twins; 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
First, fetal defects are more common in MC than in DC twin pregnancies. Second, the first‐trimester detection of fetal defects in DC twin pregnancies is similar to that in singleton pregnancies. Third, the 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.
Source: https://obgyn.onlinelibrary.wiley.com/doi/10.1002/uog.21938