The birth process, which is complex, is regulated by various hormones. When diminished levels of progesterone remove the inhibition on uterine contractions, the uterine muscles contract. The posterior pituitary gland releases the hormone oxytocin, which stimulates further contractions.
During the first stage of labor, the cervix opens and the baby descends into the birth canal, or vagina. By this time, the amniotic sac has broken. In the second stage of labor, the baby passes through the birth canal, assisted by painful uterine contractions. The baby's head normally appears first, but in a breech birth the buttocks may appear first. In the third stage of birth, the placenta (afterbirth) is delivered.