The Human Body

Pregnancy and Childbirth

Pregnancy and Childbirth, terms for the gestation period of the human reproductive cycle (see Reproduction).


Pregnancy starts when a male’s sperm fertilizes a female’s ovum (egg), and the fertilized ovum implants in the lining of the uterus (see Fertilization; Reproductive System). Because pregnancy changes a woman’s normal hormone patterns, one of the first signs of pregnancy is a missed menstrual period (see Menstruation). Other symptoms include breast tenderness and swelling, fatigue, nausea or sensitivity to smells, increased frequency of urination, mood swings, and weight gain. Some women also experience cravings for unusual substances such as ice, clay, or cornstarch; this condition, called pica, can indicate a dietary deficiency in iron or other nutrients. By the 12th week of pregnancy many of these symptoms have subsided, but others appear. For example, a woman’s breasts usually increase in size, and her nipples darken. The most obvious symptom is weight gain; most physicians now recommend a gain of about 9 to 12 kg (about 22 to 26 lb) by the end of pregnancy.

The first few months of pregnancy are the most critical for the developing infant, because during this period the infant’s brain, arms, legs, and internal organs are formed. For this reason a pregnant woman should be especially careful about taking any kind of medication except on the advice of a physician who knows that she is pregnant. X rays should also be avoided, and pregnant women should avoid smoking and alcohol consumption.


A normal pregnancy lasts about 40 weeks, or 280 days, after the beginning of the last menstrual period. Occasionally women go into labor before the expected date of birth, resulting in a premature infant. About 7 percent of all infants are premature—that is, born before the 37th week of pregnancy. Babies born just a few weeks early usually develop normally. Recent advances in the care of premature infants now allow many babies who are born after only 25 to 26 weeks of pregnancy to survive.

Delivery, the process by which the baby is expelled from the uterus through the birth canal and into the world, begins with irregular contractions of the uterus that occur every 20 to 30 minutes. As labor progresses, the contractions increase in frequency and severity. The usual length of labor for a first-time mother is about 13 to 14 hours, and about 8 or 9 hours in a woman who has given birth previously. Wide variations exist, however, in the duration of labor.

Most women prefer some kind of anesthesia to alleviate the pain associated with childbirth. Natural (unmedicated) childbirth, however, is becoming more popular, in part because many women are aware and concerned that the anesthesia and medication given to them is rapidly transported across the placenta to the unborn baby. Heavy doses of anesthesia can make the newborn baby less alert after birth.

Other options available regarding childbirth include regional (local) anesthesia, in which only those areas of the mother that are affected by the pain of childbirth are numbed. Such anesthesias include a lower spinal block and epidural anesthesia, in which the pelvic region is anesthetized. Another option is cesarean section, in which the baby is surgically removed from the uterus. Cesarean section is usually performed only for a specific medical reason.