Birth, or parturition, in mammals is the process in which a fully developed fetus is expelled from the mother's uterus by the force of strong, rhythmic muscle contractions. The birth of live offspring is a reproductive feature shared by mammals, some fishes, and selected invertebrates (such as scorpions), as well as some reptiles and amphibians. Animals who give birth to live offspring are called viviparous (meaning live birth).

In contrast to viviparous animals, other animals give birth to eggs; these animals are called oviparous (meaning egg birth). Some oviparous species, such as birds, retain their eggs inside their bodies for long periods of time; in these animals, the eggs are laid at an advanced stage of development. Other animals, such as frogs, give birth to less developed eggs, which undergo development outside the mother's body.

In both viviparous animals and oviparous animals, fertilization of the mother's egg with...