The "twin peak" sign: reliable evidence of dichorionic twinning.

In twin pregnancies with a single placental zone, the presence of a triangular projection of placental tissue beyond the chorionic surface, extending between the layers of the intertwin membrane, has provided reliable evidence that there are two fused placentas (dichorionic, diamniotic) rather than a single shared placenta (monochorionic, diamniotic). This observation, dubbed the "twin peak" sign, was identified in 15 twin pregnancies, all proven to be dichorionic at birth, and in five triplet pregnancies, all proven to be trichorionic. This finding is produced by proliferating chorionic villi growing into the potential space between the two layers of chorion in the intertwin membrane. The single chorion of a monoplacental twin pregnancy serves as an intact barrier, preventing villi from growing between the two amniotic layers. A decision sequence incorporating the use of the twin peak sign is described for determining the type of twinning that has occurred.