<P> Birth is the act or process of bearing or bringing forth offspring . In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed and breathe . In some species the offspring is precocial and can move around almost immediately after birth but in others it is altricial and completely dependent on parenting . In marsupials, the fetus is born at a very immature stage after a short gestational period and develops further in its mother's womb's pouch . </P> <P> 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, and others are viviparous, with the embryo developing inside her body, as in mammals . </P>

Animals that give birth to their young ones alive are called