In twin pregnancies prenatal diagnosis of chorionicity is important because chorionicity rather than zygocity is the main factor determining pregnancy outcome. An important feature of dichorionicity is the extension of placental tissue into the base of the inter-twin membrane. This sign has been named the Lambda sign and is typically used at 10-14 weeks (Sepulveda et al., 1996).
- Sepulveda, W., Sebire, N. J., Hughes, K., Odibo, A. and Nicolaides, K. H. (1996), The lambda sign at 10–14 weeks of gestation as a predictor of chorionicity in twin pregnancies. Ultrasound Obstet Gynecol, 7: 421–423. doi:10.1046/j.1469-0705.1996.07060421.