Amnionicity

Amnionicity is the number of amniotic sacs in a multiple gestation, most often discussed in twin pregnancies. It is considered together with chorionicity (the number of placental sacs) because the combination influences prognosis and management. The three main amnionicity patterns are diamniotic dichorionic, diamniotic monochorionic, and monoamniotic monochorionic.

Diamniotic dichorionic pregnancies have two amniotic sacs and two chorions. They are most commonly dizygotic but may also occur in some monozygotic twins if the split happens very early. The placentas are usually separate or partially fused, and the risk of cord entanglement is low compared with other patterns.

Diamniotic monochorionic pregnancies have two amniotic sacs within a single chorion. This pattern arises when a monozygotic zygote splits after the formation of the chorion but before the amnion forms. These twins share a placenta but have separate amniotic sacs, and while the risk of cord entanglement is reduced relative to monoamniotic twins, they carry risks related to sharing a placenta such as twin-to-twin transfusion syndrome.

Monoamniotic monochorionic pregnancies have one amniotic sac and one chorion, with fetuses sharing both membranes and placental circulation. This arrangement carries a high risk of cord entanglement and fetal compromise, requiring careful monitoring and often planned delivery in the third trimester.

Amnionicity is typically determined by ultrasound in early pregnancy, by identifying the dividing amniotic membrane (present in diamniotic pregnancies, absent in monoamniotic) and the chorionic pattern.