True labor commences at about 9 months (40 weeks) after the last menstrual period, resulting from the in-teraction of many factors. It is believed that both the fetus and the mother actively participate in the initi-ation of labor. For one, the stretching of the uterine wall and cervix by the enlarging fetus plays an im-portant part. In addition, the concentration of prog-esterone secreted by the placenta begins to decrease, while estrogen concentration begins to rise toward term. The fetal and maternal pituitary secretes the hormone oxytocin in increasing amounts. Oxytocin stimulates smooth muscle contraction.
Soon, strong, rhythmic contractions and true labor begins. The contractions of the uterine musculature push the baby’s head down into the pelvis, stretching the cervix and vaginal canal. The stretching of these structures stimulates sensory nerves that communi-cate with the pituitary to secrete more oxytocin by a positive feedback mechanism. This goes on until the baby is expelled from the uterus. The placenta and all the remnants are then expelled. The process of expulsion of the baby is known as parturition.
Traditionally, labor has been divided into three stages—dilation, expulsion, and placental. The di-lation stage is from the beginning of labor to the time the cervix is completely dilated. This stage lasts for about 8 hours or longer. In this stage, the amniotic sac may rupture and the fluid may leak out.
During the expulsion stage, the rate of contrac-tions occurs at about 2–3 minute intervals. It extends from the time of full cervical dilation to the expulsion of the baby and may last for about 2 hours.
During the placental stage, the uterus reduces in size and the placenta separates from the uterine wall to be expelled as the afterbirth. There is a loss of about 500–600 mL (17–20.3 oz) of blood at this time.
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