[Ultrasonic diagnosis of chorionicity in multiple pregnancies].
The management of multiple pregnancies requires a correct diagnosis of chorionicity. This assessment is easy and reliable before 15 weeks of gestation by ultrasonography, but becomes much more difficult during the second and third trimester for same sex foetuses. From 5 to 8 weeks of gestation, the visualization of two gestational sacs assesses bichorionicity. From 8 to 14 weeks, the diagnosis of chorionicity is based on the presence of the lambda sign, completed by the evaluation of the thickness of the inter-twin membrane and the placenta localisation. From 15 weeks onward, the twin peak (or lambda) sign remains the best predictor of dichorionicity but is valuable only if it is present. The measurement of the thickness of the inter-twin membrane and the count of its layers are not available in all cases. From March 1999 to March 2000, we studied 31 multiple pregnancies with same sex foetuses, referred to our centre during the second trimester. The patients were asked for their former ultrasound reports. Chorionicity was mentioned only in 77% of the cases and when mentioned the information was correct in 85% of the cases. Thus, improving on that point is necessary. Prospective studies focusing on ultrasound determination of chorionicity show accuracy close to 100% when the ultrasonography is correctly realized before 15 weeks of gestation.