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Ultrasound Obstet Gynecol. 1996 Jun;7(6):421-3.

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

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Harris Birthright Research Centre for Fetal Medicine, King's College Hospital Medical School, London, UK.


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.

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