Parturition is childbirth. Estrogens increase dramatically near the time of birth, as does corticotropin-releasing hormone (CRH).
Labor consists of three stages: dilation, expulsion, and placental. Contractions begin at the top of the uterus and sweep in a wave toward the cervix. When labor begins, oxytocin is released from the pituitary in response to cervical stretch (positive feedback). In the dilation stage, the fetus shifts toward the cervix which dilates to 10 cm and the amniotic sac bursts. The expulsion stage refers to the actual delivery of the baby with its umbilical cord attached. The placental stage is the later delivery of the placenta, amniotic sac and attached umbilical cord.
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