Prenatal ultrasonographic assessment of fetal sex and placental/membrane morphology is highly accurate in predicting chorionicity. The lambda sign, an echogenic chorionic tissue projection into the base of the intertwin membrane, is evident in all dichorionic pregnancies at 10–14 weeks’ gestation but disappears by the 20th week in about 7% of dichorionic pregnancies with fused placenta. Monochorionic twins should be scanned every 2 weeks to look for evidence of transfusion pathology. Dichorionic twins with normal biometry, umbilical Doppler values and amniotic fluid levels should be scanned every 4–6 weeks from 24 weeks onwards to monitor growth and well-being.
Source: Forfar and Arneil’s Textbook of Pediatrics, 7E