But twin pregnancies are obvious evidence that it's possible to release more than one egg per menstrual cycle, and the incidence of twin deliveries gives some clue as to how often this happens. About 1 in 80 deliveries involves the birth of spontaneously conceived twins. Monozygotic twins result when a single fertilized egg splits after conception. This egg divides and becomes two individuals who will share all of their genes in common and who are essentially genetic clones of one another. The incidence of monozygotic twins is about 1 in 250 deliveries, and it is the same all over the world.
Dizygotic twins result when two separate eggs are fertilized.
The babies borne of these eggs are not necessarily any more alike than any two children born to the same parents. Fraternal twins are the most common type, and they may or may not be of the same sex. The incidence of dizygotic twin births is affected by heredity, race, maternal age, and number of previous children. It is highest in women of African descent and lowest in women of Asian descent. The tendency to bear fraternal twins may be inherited, and these are the types of twins that can run in the family.