The lambda sign at 10–14 weeks of gestation as a predictor of chorionicity in twin pregnancies

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Abstract
Chorionicity was prospectively determined in 369 twin pregnancies by ultrasound at 10–14 weeks of gestation. Pregnancies were classified as monochorionic if there was a single placental mass in the absence of the lambda sign at the inter‐twin membrane‐placental junction, and dichorionic if there was a single placental mass but the lambda sign was present, or the placentas were not adjacent to each other. In 81 (22%) cases, the pregnancies were classified as monochorionic and in 288 (78%) as dichorionic. Pregnancy outcome was available in 279 cases and all 63 of these pregnancies classified as monochorionic resulted in the delivery of same‐sex twins. Similarly, all 100 different‐sex pairs were correctly classified as dichorionic. These findings demonstrate the high reliability of ultrasound examination at 10–14 weeks of gestation in determining chorionicity in twin pregnancies. Copyright © 1996 International Society of Ultrasound in Obstetrics and Gynecology

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