Because fraternal, or dizygotic, twins are 2 separate fertilized eggs, they usually develop 2 separate amniotic sacs, placentas, and supporting structures. Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2.

What happens when twins share the same sac?

Twins who share the same amniotic sac, a condition that occurs in less than 1 percent of all U.S. twin pregnancies, face serious risks — including cord entanglement, which can cut off the blood flow from the placenta to the fetus.

Can identical twins have separate sacs and placentas?

About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas. ... In this case, both twins share an inner sac, called the amnion, in addition to sharing a placenta.