| Themes > Science > Life Sciences > General Biology > Physiology > The Reproductive System > Birth |
Birth is a positive feedback hormonal mechanism. During birth the cervix dilates to allow passage of the fetus. Uterine contractions propel the fetus through the birth canal, usually head first. Hormonal control of the birth process involves the release of oxytocin and prostaglandins, which are stimulated by uterine contractions, which stimulate more hormones that cause more contractions....etc. First Stage The first stage of birth lasts from beginning of contractions to the full (10 cm) dilation of the cervix. Membranes of the amniotic fluid rupture, lubricating the vagina. Second Stage Strong uterine contractions of a minute in duration separated by two to three minute intervals propel the fetus down the birth canal. Abdominal muscles relax in synchrony with the uterine contractions. Third Stage After delivery of the baby, the umbilical cord is clipped and cut. The placenta (or afterbirth) in expelled through the vagina. Milk Production Nursing mothers have their hormone levels and uterine size return to normal much faster than non-nursing mothers. Breasts develop the capability for milk secretion about the mid point of pregnancy. Secretion of milk does not occur until delivery, and the action of prolactin. Suckling by the infant causes production of oxytocin to promote release of milk into the ducts emptying into the nipple. |
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