Identical, or monozygotic twins, develop from the same fertilized ovum. The blastomeres may separate early in cleavage, or the inner cell mass may split prior to gastrulation. Identical twins have the same genetic makeup because they are formed from the same pair of gametes. They look alike and are the same sex.

Identical twins will often share the same placenta, but usually have separate amniotic sacs. Each twin always has its own, separate umbilical cord. Nonidentical twins have separate placentas, amniotic sacs, and umbilical cords.