Understanding Dichorionic and Diamniotic Twins in Monozygotic Twins

Dichorionicity and diamniosis occur when fraternal (dizygotic) twins develop from two separate fertilized eggs that individually implant into the uterine wall. Because the twins develop from separate fertilized eggs, each has its chorionic sac and amniotic sac.

Monozygotic (MZ) twins, which also are called identical twins, occur when a single fertilized egg splits in half forming two separate embryos that implant into the uterus. Because the twins share the same fertilized egg, they have a single chorionic sac and a single amniotic sac surrounding them both.