How multiples are made?

:womenshealth.gov

2012/08/22

804

Fraternal twins — the most common kind — occur when two separate eggs are fertilized by two different sperm. Each twin has his or her own placenta and amniotic sac. The twins can be two girls, two boys, or a boy and a girl. Genetically, they're no more alike than any other siblings.

Identical twins occur when a single fertilized egg splits and develops into two fetuses. Identical twins might share a placenta, but each baby usually has a separate amniotic sac. Genetically, the two babies are identical. They'll be the same sex and share physical traits and characteristics. Rarely, identical twins fail to completely separate into two individuals. These babies are known as conjoined twins.

Triplets and other higher order multiples can be identical, fraternal or a combination of both.