There are two distinct types of twins in humans: identical and fraternal. Identical twins, also called monozygotic twins, result from the splitting of a single zygote into two distinct embryos. Because they arise from a single fertilized egg, monozygotic twins are always of the same sex, and being genetically identical, they tend to bear remarkable similarities in physical appearance. Identical twinning occurs in roughly 1/250 pregnancies worldwide, with relatively little variation between different populations. Evidence indicates that there is no tendency toward identical twin births in families (i.e., it does not "run in families").

Fraternal twins result from the fertilization of two different eggs by two different sperm during the same ovulatory cycle. Fraternal twins, also called dizygotic twins, can be of the same sex, or one of each sex, and are no more alike genetically than other siblings. Fraternal twinning varies in frequency around the world with...