Monochorionic Twins: Identical vs. Fraternal – What You Need to Know

Not necessarily. Twins who share the same placenta are called monochorionic twins. Monochorionic twins can be either identical (monozygotic) or fraternal (dizygotic). Identical twins occur when a single fertilized egg splits into two embryos, while fraternal twins occur when two separate eggs are fertilized by two different sperm.

Monochorionic identical twins share the same placenta and the same chorionic sac, which is the outer membrane that surrounds the placenta and the fetus. They also share the same amniotic sac, which is the fluid-filled sac that surrounds the fetus. Monochorionic fraternal twins share the same placenta but have separate chorionic sacs and amniotic sacs.

Most monochorionic twins are identical, but a small percentage are fraternal. The type of twins can be determined by genetic testing.