the sequence of actions by which a baby and the afterbirth (placenta) are expelled from the uterus at childbirth. The process usually starts spontaneously about 280 days after conception, but it may be started by artificial means (see induction). In the first stage the muscular wall of the uterus begins contracting while the muscle fibres of the cervix relax so that the cervix expands. A portion of the membranous sac (amnion) surrounding the baby is pushed into the opening and ruptures under the pressure, releasing amniotic fluid to the exterior. In the second stage the baby's head appears at the cervix and the contractions of the uterus strengthen. The passage of the infant through the vagina is assisted by contractions of the abdominal muscles and conscious pushing by the mother. When the top of the baby's head appears at the vaginal opening the whole infant is eased clear of the vagina, and the umbilical cord is cut. If the emergence of the head is impeded an incision may be made in the surrounding tissue (see episiotomy). In the final stage the placenta and membranes are pushed out by the continuing contraction of the uterus, which eventually returns to its unexpanded state. The average duration of labour is about 12 hours in first pregnancies and about 8 hours in subsequent pregnancies. The pain of labour may be reduced if the mother trains her abdominal muscles during the prenatal period and by the use of drugs (accelerated labour). See also Caesarean section.

Medical dictionary. 2011.

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