Monochorionic Diamniotic Twin Gestations

Abstract The term dichorionic refers to a multiple gestation with two distinct placental disks (or two chorions); the term diamniotic describes a pregnancy with two distinct amniotic cavities. Either monozygotic or dizygotic twinning can result in a dichorionic diamniotic twin gestation. In the early first trimester, dichorionic diamniotic twin pregnancies can be identified by the presence of two distinct gestational sacs separated by a thick dividing membrane, each containing a separate fetal pole, yolk sac, and amniotic sac. Dichorionic pregnancies have a thick intertwin membrane and a characteristic thick triangular projection of placenta between the layers of the dividing membrane known as the twin peak or lambda sign. The finding of two distinct placental masses each with a cord insertion or the finding of discordant fetal gender is almost always diagnostic of a dichorionic twin gestation. Prenatal ultrasound (US) is essential in the management of dichorionic twin pregnancies to allow determination of chorionicity, evaluation of fetal anatomy, and serial US assessment of cervical length and fetal growth.