OBJECTIVE To study the sonographic evolutaion of the chorionic tissue projection at the intertwin membrane-placental junction, or lambda sign, or twin-chorionic peak sign, in dichorionic twin pregnancies and to determine the effect of placental location on its prevalence during the first half of pregnancy. METHODS We prospectively recruited women with twin pregnancies undergoing sonographic screening for chromosomal abnormalities at 10-14 weeks' gestation for this study. The presence or absence of the lambda sign and the position of the placenta(s) were recorded in all cases. The presence of the lambda sign was evaluated prospectively at 16 and 20 weeks' gestation. RESULTS There were 101 twin pregnancies with a lambda sign identified at 10-14 weeks' gestation, 67 with fused placentas and 34 with separate placentas. At 16 weeks, the lambda sign was present in all 67 pregnancies with fused placentas (100%) and in 31 of 34 (91%) pregnancies with separate placentas. At 20 weeks, the lambda sign was present in 25 of the 34 (74%) pregnancies with separate placentas and in 62 of the 67 (93%) with fused placentas. The lambda sign was identified subsequently in none of the 53 pregnancies in which it was absent at 10-14 weeks' gestation. CONCLUSIONS At 10-14 weeks' gestation, twin pregnancies with the lambda sign can be classified as dichorionic and pregnancies with absent lambda sign can be classified as monochorionic and therefore monozygotic. At 16-20 weeks, the lambda sign is indicative of dichorionicity but its absence does not exclude dizygosity.