During the latter stages of pregnancy, the blood concentration of estrogens become increasingly greater than that of progesterone, as noted earlier. Whereas progesterone inhibits uterine contractions, estrogens promote them. Therefore, there is an increasing tendency toward the onset of uterine contractions as the pregnancy approaches full term. These uterine contractions are often referred to as Braxton Hicks contractions or “false labor.” Throughout pregnancy, the hormone relaxin is secreted first by the corpus luteum and then by the placenta. It helps in the development of blood vessels within the placenta, in addition to other cardiovascular changes that occur in the mother.
Birth usually occurs within two weeks of the calculated due date, which is 280 days from the beginning of the last menstruation. The birth process is called parturition (par-tu-rish’-un), and the events associated with parturition are collectively called labor. The fetus is usually in a “head down” position at this time. As the fetus reaches full term, the high blood levels of estrogens override progesterone’s inhibition of uterine contractions, allowing uterine contractions to occur. Pressure of the fetus on the cervix stretches the cervix, stimulating a neuroendocrine positive-feedback mechanism that promotes uterine contractions. In fact, physicians sometimes initiate labor by breaking the amnion so that increased pressure is placed on the cervix. The process of labor can be divided into three stages.
The first stage of labor is the dilation of the cervix. The stretching of the cervix triggers the formation of nerve impulses that are sent to the hypothalamus. When these reach a critical frequency, the hypothalamus activates the posterior lobe of the pituitary gland to release oxytocin, increasing its concentration in the blood. Oxytocin stimulates the characteristic rhythmic contractions that begin at the superior end of the uterus and move toward the cervix, pushing the fetus toward the vagina, or birth canal.
The continued contractions of the uterus force the fetus’s head against the cervix, which results in greater stretching of the cervix. The increase in cervical stretching causes more nerve impulses to be sent to the hypothalamus. The hypothalamus then stimulates the posterior lobe of the pituitary to release more oxytocin into the blood. The higher blood levels of oxytocin trigger more intense and frequent uterine contractions to occur, which in turn produce greater cervical stretching. This positive- feedback mechanism will produce increasingly stronger uterine contractions until birth occurs.
Dilation of the cervix is the longest stage of labor. It may last from 6 to 12 hours, depending on the size of the fetus and whether the mother has had other children. During this time, the amnion ruptures (or “water breaks”) and the cervix dilates to the size of the fetus’s head.
The second stage of labor is the delivery (expulsion) of the fetus. It usually lasts less than an hour, with contractions occurring every two to three minutes and lasting about one minute. Once the head is expelled, the rest of the body exits rather quickly.
The third stage of labor is the delivery of the placenta. Within 15 minutes after birth of the infant, the placenta detaches. Continued contractions expel the placenta (the afterbirth). The placenta is checked carefully to see that all of it has been removed from the uterus because any residue may cause a serious uterine infection. Detachment of the placenta produces some bleeding because endometrial blood vessels at the placental site are ruptured. However, uterine contractions compress the broken blood vessels so that serious bleeding is usually avoided. Subsequently, the uterus decreases in size rather quickly.
Immediately after birth, the infant’s nose and mouth are aspirated to remove mucus or fluid that would impair breathing. The umbilical cord is clamped and cut, separating the infant from the placenta, which has served as its prenatal respiratory organ. As carbon dioxide increases in the infant’s blood, the respiratory rhythmicity center in the medulla oblongata is activated and the infant’s first inspiration is stimulated.
The first breath is difficult because the lungs are collapsed. In an infant, surfactant in alveoli reduces surface tension, making the first breath and subsequent breathing easier.