"Birth is that the act or process of bearing or bringing forth offspring also mentioned in technical contexts as parturition. In mammals, the method is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it's ready to feed and breathe. In some species, the offspring is precocial and may move around soon after birth but in others, it's altricial and completely hooked into parenting. In marsupials, the fetus is born at a really immature stage after a brief gestational period and develops further in its mother's womb's pouch.
It is not only mammals that give birth. Some reptiles, amphibians, fish, and invertebrates carry their developing young inside them. Some of these are ovoviviparous, with the eggs being hatched inside the mother's body, et al. are viviparous, with the embryo developing inside her body, as within the case of mammals."