First-trimester ultrasound determination of chorionicity in twin pregnancy

Objective To determine the accuracy of ultrasound at 11-14 weeks' gestation in the diagnosis of chorionicity in twin pregnancy. Methods This was a retrospective observational study of data obtained between 1999 and 2010. At the first-trimester routine ultrasound scan, chorionicity was assigned according to the number of placental masses and T or lambda-signs for a single placental mass. Chorionicity was confirmed by histology or discordant sex at birth. Results A total of 648 pregnancies were assigned chorionicity by first-trimester ultrasound during the study period. Chorionicity was ascertained in 613 cases, either by histology (n = 340) or discordant sex (n = 273). Chorionicity was correctly assigned by ultrasound at 11-14 weeks in 612 of 613 pregnancies (accuracy 99.8%). Sensitivity and specificity for determining monochorionicity were 100% and 99.8%, respectively. Conclusions First-trimester ultrasound can be used to determine chorionicity reliably by noting the number of placental masses and T or lambda-signs. Determination of twin chorionicity is important and should be completed in the first trimester. Copyright (C) 2011 ISUOG. Published by John Wiley & Sons, Ltd.