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Birth, the climax of gestation (the development of a child or other baby mammal within its mother's body) and the beginning of an independent life. In humans, a normal birth proceeds in 3 stages. Mild labor pains caused by contractions of the muscles of the uterus are usually the first sign that a woman is about to give birth. The contractions push the baby downwards, usually head first, which breaks the membranes surrounding the baby, causing the amniotic fluid to escape. In the second stage of labor, stronger contractions push the baby through the cervix and vagina, or birth canal. This is the most painful part and usually lasts less than 2 hours. Anesthetics and analgesics are commonly administered at this time, and delivery is aided by hand or with obstetric forceps. In some cases, the baby must be delivered by a surgical procedure called cesarean section. As soon as the baby is born, its nose and mouth are cleared of fluid and breathing starts, whereupon the umbilical cord is cut and tied. In the third stage of labor, the placenta is expelled from the uterus and bleeding is stopped by further contractions.The exact mechanism by which labor is initiated remains unknown; however, recent research indicates that hormones both from the placenta and from the mother's pituitary gland play important roles in the onset of labor.

See also: Reproduction.

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