Multiple birth is the delivery of more than one child at the end of pregnancy. Twins, the commonest type of multiple birth, are of two distinct varieties. Monozygotic or identical twins originate in a single fertilized egg (zygote) which divides, each half (containing identical genetic material) developing independently into an embryo and fetus, although they may share a common placenta. Dizygotic or non-identical twins originate in the release of two eggs at ovulation (see menstruation), each being fertilized, implanting, and developing separately. There is no more relation between their genes than between those of other siblings. Higher orders of multiple births (triplets, quadruplets, quintuplets, etc) usually arise from multiple ovulation and are rare unless ovarian follicle stimulants (e.g., gonadotrophins) have been used in the treatment of infertility; here the dosage is critical. Multiple pregnancy may run in families. Prematurity, toxemia, anemia, and other complications are more common in multiple pregnancy.