They have separate placentas, although sometimes the placentas may grow together. Women who become pregnant soon after stopping the birth control pill. Women who have many family members with twins. Thirty percent of twins are identical. They are formed by the division of a single fertilized egg.
Identical twins are the same sex and the same genetic makeup.
Before the twins are born, they may have the same sac as well as the same placenta or they may have separate sacs and placentas. Sharing a placenta or a sac causes special problems for the pregnancy. When twins share the same sac, they are called monoamniotic. These twins always share the same placenta. These are the rarest types of twins When twins share the same placenta they are called monochorionic.