PARTURITION

also called birth, or childbirth process of bringing forth a child from the uterus, or womb. (The prior development of the child in the womb is described in the article human embryology.) also called birth or childbirth the process of bringing forth an infant from the uterus. Childbirth in humans is commonly divided into three stages. The first stage, dilatation, is characterized by the opening up of the cervix until it is about 4 inches (10 cm) in diameter, large enough for an infant's head to pass through. Rhythmic muscular contractions of the uterus force the sac of amniotic fluid, which has held the fetus throughout pregnancy, down into the cervix and compress its contents until it bursts. At the onset of labour, uterine contractions may occur every 20 or 30 minutes and last about 40 seconds. As labour progresses, the contractions increase in frequency and in intensity. When the cervical opening is fully dilated, contractions may recur every two to three minutes. The period of dilatation varies greatly; in women having their first baby it may vary from a few hours to (intermittently) several days, while in those who have borne several children it may require less than an hour. In the second stage of labour, expulsion, the baby is pushed through the birth canal. Contractions generally occur about every two minutes and last 60 seconds or longer. In normal deliveries, the crown of the baby's head emerges first, followed by the shoulders, one at a time. The lower part of the baby's body then slides out quickly. An episiotomy (an incision in the vulvar orifice) is sometimes performed to prevent the mother's perineal tissues from tearing. In a normal delivery, the expulsion stage may last anywhere from a few minutes to one or two hours. Expulsion may be complicated by the incorrect positioning of the baby for delivery. In such cases, the child's position can be changed manually or with forceps. Sometimes, as when it is lying transversely across the mother's pelvis, the baby must be delivered surgically. The third stage of labour begins when the birth of the child is complete and ends when the placenta is delivered. The placental stage usually lasts less than 15 minutes. The pain of labour can be relieved by the use of analgesic drugs, but these pose some degree of danger to the mother and baby and must be used carefully. If the mother is unconscious owing to a general anesthetic, she will not be able to augment the uterine contractions by bearing down, nor will she experience the birth of the child. Local anesthetics in the genital and pelvic region are sometimes used instead. The technique of natural childbirth was developed in the 1940s to reduce dependence on anesthesia. Because fear and tension exacerbate labour pain, pregnant women are often educated in the process of childbirth. In addition, they are taught exercises to strengthen the muscles involved in labour and breathing techniques to aid relaxation and reduce pain. If a difficult delivery is expected, natural childbirth is not encouraged. Several factors may lead to complications in childbirthe.g., abnormal bleeding, placenta previa (expulsion of the placenta before the child), knotting of the umbilical cord, failure of the mother's blood to clot, weak uterine contractions, and rupture of the uterus. If delivery through the birth canal is likely to prove hazardous to the mother or the child, a cesarean section is performed. This procedure involves making an incision in the mother's abdomen through which the baby is removed. The type of incision that is involved in a modern cesarean section and the scar it leaves in the uterine wall usually do not prevent a woman from delivering naturally in subsequent births. Additional reading Francine H. Nichols and Sharron Smith Humenick, Childbirth Education: Practice, Research, and Theory (1988); Oxorn-Foote Human Labor & Birth, 5th ed. by Harry Oxorn (1986); Helen Varney, Nurse-Midwifery, 2nd ed. (1987). The Editors of the Encyclopdia Britannica Puerperium Within six to eight weeks after childbirth, most of the structures of the maternal organism that underwent change during pregnancy return more or less to their prepregnancy state. The enlarged uterus, which at the end of gestation weighed about 1,000 grams (35 ounces), shrinks to a weight of about 60 grams (2 ounces). Along with this process of uterine involution, the lining membrane of the uterus is almost completely shed and replaced by a new lining, which is then (six to eight weeks after delivery) ready for the reception of another fertilized ovum (egg). The greatly dilated neck of the womb and lower birth passage likewise undergo marked and rapid involution, but they seldom return exactly to their prepregnancy condition. As a rule, examination of a woman who has given birth accordingly reveals evidence of this. The markedly stretched abdominal wall also undergoes considerable involution, particularly if abdominal exercises are carried out. Although the intradermal tears (striae gravidarum) become smaller and fade, they do not completely disappear but remain as evidence of the marked and rapid stretching of the skin that took place during pregnancy. John W. Huffman