Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. 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 gestation and develops further in its mother's womb pouch.
It is not only mammals that give birth.