Sensitivity of first-trimester ultrasound in the detection of congenital anomalies in twin pregnancies: population study and systematic review.

First-trimester detection of structural abnormalities in twin pregnancies is possible in 27.3% (95% confidence interval 15.0-42.8) of cases. The likelihood for first-trimester detection of structural anomalies in twins was maximum for cranial vault, midline brain and abdominal wall defects. Monochorionicity and increasing discrepancy in crown-rump length and nuchal translucency were associated with fetal structural abnormalities, although their predictive performance was only moderately good.