Twins

The incidence of twins is about 1 in 80 births; two-thirds of twin births are dizygotic and only one-third is monozygotic.

Dizygotic twins

Dizygotic twins arise if two oocytes are each fertilized by a sperm cell. Each embryo generally develops its own amnion and chorion, and placenta. However, it is possible that the chorionic and even the amniotic cavities can fuse together, and a common placenta is formed.