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Near the time of birth, the amniotic sac — a fluid-filled membrane that encloses the fetus within the uterus — breaks in a gush of fluid. This is commonly called “breaking water.” Labor usually begins within a day of this event, although it may begin prior to it. Labor is the general term for the process of childbirth in which regular uterine contractions push the fetus and placenta out of the body. Labor can be divided into three stages, which are illustrated in Figure 22.8.5 : dilation, birth, and after birth.
During the dilation stage of labor, uterine contractions begin and become increasingly frequent and intense. The contractions push the baby’s head (most often) against the cervix, causing the cervical canal to dilate, or become wider. This lasts until the cervical canal has dilated to about 10 cm (3.9 in.) in width, which may take 12 to 20 hours — or even longer. The cervical canal must be dilated to this extent in order for the baby’s head to fit through it.
During birth, the baby descends (usually headfirst) through the cervical canal and vagina, and into the world outside. This is the stage when the mother generally starts bearing down during the contractions to help push out the fetus. This stage may last from about 20 minutes to two hours or more. Usually, within a minute or less of birth, the umbilical cord is cut, so the baby is no longer connected to the placenta.
During the afterbirth stage, the placenta is delivered. This stage may last from a few minutes to a half hour.