Adjustments to Pregnancy

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Pregnancy places a considerable stress on a woman's body and requires adjustments in nearly all the organ systems. A few of the major adjustments and effects of pregnancy are described here.

Digestive System, Nutrition, and Metabolism

For many women, one of the first signs of pregnancy is morning sickness—nausea, especially after rising from bed, in the first few months of gestation. The cause of morning sickness is unknown. One hypothesis is that it stems from the reduced intestinal motility caused by the steroids of pregnancy. Another is that it is an evolutionary adaptation to protect the fetus from toxins. The fetus is most vulnerable to toxins at the same time that morning sickness peaks. Women with morning sickness tend to prefer bland foods and to avoid spicy and pungent foods, which are highest in toxic compounds. In some women, the nausea progresses to vomiting.

Constipation and heartburn are common in pregnancy. The former is another result of reduced intestinal motility. The latter is due to the enlarging uterus pressing upward on the stomach, causing the reflux of gastric contents into the esophagus.

The basal metabolic rate rises about 15% in the second half of gestation. Pregnant women often feel overheated because of this and the effort of carrying the extra weight. The appetite may be strongly stimulated, but a pregnant woman needs only 300 extra Calories per day even in the last trimester. With poor prenatal care and little self-control, however, some women greatly overeat and gain as much as 34 kg (75 lb) of weight compared with a healthy average of 11 kg (24 lb). Maternal nutrition should emphasize the quality of food eaten, not quantity.

During the last trimester, the fetus needs more nutrients than the mother's digestive tract can absorb. In preparation for this, the placenta stores nutrients early in gestation and releases them in the final trimester. The demand is especially high for protein, iron, calcium, and phosphates. A pregnant woman needs an extra 600 mg of iron for her own hemopoiesis and 375 mg for the fetus. She is likely to become anemic if she does not ingest enough iron during late pregnancy. Supplemental vitamin K is often given late in pregnancy to promote prothrombin synthesis in the fetus. This reduces the risk of neonatal hemorrhage, especially in the brain, caused by the stresses of birth. Supplemental folic acid reduces the risk of neurological disorders in the fetus, such as spina bifida (see insight 13.1, p. 484) and anencephaly (failure of the cerebrum, cerebellum, and calvaria to develop). A vitamin D supplement helps to ensure adequate calcium absorption to meet fetal demands.

Circulatory System

By full term, the placenta requires about 625 mL of blood per minute from the mother. The mother's blood volume rises about 30% during pregnancy because of fluid retention and hemopoiesis; she eventually has about 1 to 2 L of extra blood. Cardiac output rises about 30% to 40% above normal by 27 weeks, but for unknown reasons, it falls almost to normal in the last 8 weeks. As the pregnant uterus puts pressure on the large pelvic blood vessels, it interferes with venous return from the legs and pelvic region. This can result in hemorrhoids and varicose veins.

Respiratory System

Minute ventilation increases about 50% during pregnancy for two reasons: (1) Oxygen demands are about 20% higher by late pregnancy in order to supply the fetus and support the woman's increased metabolic rate. (2) Progesterone increases the sensitivity of her respiratory chemoreceptors to carbon dioxide, and ventilation is adjusted to keep her arterial PCO2 lower than normal. While there is a demand for increased ventilation, the expanding uterus pushes the abdominal viscera up against the diaphragm and interferes with breathing. Consequently, the respiratory rate increases to compensate for the lack of depth. Pressure on the diaphragm may be great enough to cause breathing difficulty (dyspnea) by late pregnancy. In the last month, however, the pelvis usually expands enough for the fetus to drop lower in the abdominopelvic cavity, taking some pressure off the diaphragm and allowing the woman to breathe more easily.

Urinary System

Aldosterone and the steroids of pregnancy promote water and salt retention by the kidneys. Nevertheless, the glomerular filtration rate increases by 50% and urine output is slightly elevated. This enables a woman to dispose of both her own and the fetus's metabolic wastes. As the pregnant uterus compresses the bladder and reduces its capacity, urination becomes more frequent and some women experience uncontrollable leakage of urine, or incontinence.

Integumentary System

The skin must grow to accommodate expansion of the abdomen and breasts and the added fat deposition in the hips and thighs. Stretching of the dermis often tears the connective tissue and causes striae, or stretch marks. These appear reddish at first but fade after pregnancy. Melanocyte activity increases in some areas and darkens the areolae and linea alba. The latter often becomes a dark

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Chapter 28 The Female Reproductive System 1073

line, the linea nigra38 (LIN-ee-uh NY-gruh), from the umbilical to the pubic region. Some women also acquire a temporary blotchy darkening of the skin over the nose and cheeks called the "mask of pregnancy," or chloasma39 (clo-AZ-muh), which usually disappears when the pregnancy is over.

Uterine Growth and Weight Gain

The uterus weighs about 50 g when a woman is not pregnant and about 900 g by the end of pregnancy. Its growth is monitored by palpating the fundus, which eventually reaches almost to the xiphoid process (fig. 28.19). Table 28.3 shows how the weight gained in pregnancy is distributed.

linea = line + nigra = black 39chloasma = to be green

Chapter 28 The Female Reproductive System 1073

Table 28.3 Distribution of Weight Gain in Pregnancy


3 kg (7 lb)

Placenta, fetal membranes, and amniotic fluid

1.8 kg (4 lb)

Blood and tissue fluid

2.7 kg (6 lb)


1.4 kg (3 lb)


0.9 kg (2 lb)


0.9 kg (2 lb)


11 kg (24 lb)


In the seventh month of gestation, the fetus normally turns into a head-down vertex position. Consequently, most babies are born head first, the head acting as a wedge that

Full Term Pregnancy Viscera
Figure 28.19 The Full-Term Fetus in Vertex Position.

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Physiology: The Unity of Reproductive System Companies, 2003 Form and Function, Third Edition

1074 Part Five Reproduction and Development widens the mother's cervix, vagina, and vulva during birth. The ancients thought that the fetus kicks against the uterus and pushes itself out head first. The fetus, however, is a rather passive player in its own birth; its expulsion is achieved only by the contractions of the mother's uterine and abdominal muscles. Yet there is evidence that the fetus may play some role in its birth by chemically stimulating labor contractions and perhaps even sending chemical messages that signify when it is ready to be born.

Uterine Contractility

Over the course of gestation, the uterus exhibits relatively weak Braxton Hicks40 contractions. These become stronger in late pregnancy and often send women rushing to the hospital with "false labor." At term, however, these contractions transform suddenly into the more powerful labor contractions. True labor contractions mark the onset of parturition (PAR-too-RISH-un), the process of giving birth.

Progesterone and estrogen balance may be one factor in this pattern of increasing contractility. Both hormone levels increase over the course of gestation. Progesterone inhibits uterine contractions, but its secretion levels off or declines slightly after 6 months, while estrogen secretion continues to rise (see fig. 28.18). Estrogen stimulates uterine contractions and may be a factor in the irritability of the uterus in late pregnancy.

Also, as the pregnancy nears full term, the posterior pituitary releases more oxytocin (OT) and the uterus produces more OT receptors. Oxytocin promotes labor in two ways: (1) it directly stimulates muscle of the myometrium, and (2) it stimulates the fetal membranes to secrete prostaglandins, which are synergists of OT in producing labor contractions. Labor is prolonged if OT or prosta-glandins are lacking, and it may be induced or accelerated by giving a vaginal prostaglandin suppository or an intravenous OT "drip." The conceptus itself may produce chemical stimuli promoting its own birth. Fetal cortisol secretion rises in late pregnancy and may enhance estrogen secretion by the placenta. The fetal pituitary gland also produces oxytocin, which does not enter the maternal circulation but may stimulate the fetal membranes to secrete prostaglandins.

Uterine stretching is also thought to play a role in initiating labor. Stretching any smooth muscle increases its contractility, and movements of the fetus produce the sort of intermittent stretch that is especially stimulatory to the myometrium. Twins are born an average of 19 days earlier than single infants, probably because of the greater stretching of the uterus. When the fetus is in the vertex position, its head pushes against the cervix, which is especially sensitive to stretch.

40John Braxton Hicks (1 823-97), British gynecologist

Labor Contractions

Labor contractions begin about 30 minutes apart. As labor progresses, they become more intense and eventually occur every 1 to 3 minutes. It is important that they be intermittent rather than one long, continual contraction. Each contraction sharply reduces maternal blood flow to the placenta, so the uterus must periodically relax to restore flow and oxygen delivery to the fetus. Contractions are strongest in the fundus and body of the uterus and weaker near the cervix, thus pushing the fetus downward.

According to the positive feedback theory of labor, labor contractions are induced by stretching of the cervix. This triggers a reflex contraction of the uterine body that pushes the fetus downward and stretches the cervix still more. Thus there is a self-amplifying cycle of stretch and contraction. In addition, cervical stretching induces a neuroendocrine reflex through the spinal cord, hypothalamus, and posterior pituitary. The posterior pituitary releases oxytocin, which is carried in the blood and stimulates the uterine muscle both directly and through the action of prostaglandins. This, too, is a positive feedback cycle: cervical stretching ^ oxytocin secretion ^ uterine contraction ^ cervical stretching (see fig. 1.13, p. 19).

As labor progresses, a woman feels a growing urge to "bear down." A reflex arc extends from the uterus to the spinal cord and back to the skeletal muscles of the abdomen. Contraction of these muscles—partly reflexive and partly voluntary—aids in expelling the fetus.

The pain of labor is due at first mainly to ischemia of the myometrium—muscle hurts when deprived of blood, and each labor contraction temporarily restricts uterine circulation. As the fetus enters the vaginal canal, the pain becomes stronger because of increased stretching of the cervix, vagina, and perineum and sometimes the tearing of vaginal tissue. At this stage, the obstetrician may perform an episiotomy—an incision in the vulva to widen the vaginal orifice and prevent random tearing. The pain of human childbirth, compared to the relative ease with which other mammals give birth, is an evolutionary product of two factors: the unusually large brain and head of the human infant, and the narrowing of the pelvic outlet, which adapted hominids to bipedal locomotion (see p. 286).

Stages of Labor

Labor occurs in three stages. The duration of each stage tends to be longer in a primipara (a woman giving birth for the first time) than in a multipara (a woman who has previously given birth).

Dilation (First) Stage This is the longest stage, lasting 8 to 24 hours in a primipara but as little as a few minutes in a multipara. It is marked by the dilation (widening) of the cervical canal and effacement (thinning) of the cervix (fig. 28.20a, b). The cervix reaches a maximum diameter of about 10 cm (the diameter of the baby's head). During

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Positive Feedback Childbirth
Figure 28.20 The Stages of Childbirth. (a) Early dilation. (b) Late dilation. (c) Expulsion. (d) The placental stage.

dilation, the fetal membranes usually rupture and the amniotic fluid is discharged (the "breaking of the waters").

Expulsion (Second) Stage This stage typically lasts about 30 to 60 minutes in a primipara and as little as 1 minute in a multipara. It begins when the baby's head enters the vagina and lasts until the baby is entirely expelled (fig. 28.20c). The baby is said to be crowning when the top of its head is visible, stretching the vulva (fig. 28.21a). Delivery of the head is the most difficult part, with the rest of the body following much more easily. An episiotomy may be performed during this stage. Women in labor employ the Valsalva maneuver—holding the breath while contracting the abdominal muscles—to help expel the infant. An attendant often uses a suction bulb to remove mucus from the baby's mouth and nose even before it is fully delivered. When the baby is fully expelled, an attendant drains the blood of the placental vein into the baby, clamps the umbilical cord in two places, and cuts the cord between the clamps.

Placental (Third) Stage The uterus continues to contract after expulsion of the baby. The placenta, however, is a nonmuscular organ that cannot contract, so it buckles away from the uterine wall (see fig. 28.20d). About 350 mL of blood is typically lost at this stage, but contractions of the myometrium compress the blood vessels and prevent more extensive bleeding. The placenta, amnion, and other fetal membranes are expelled by uterine contractions, which may be aided by a gentle pull on the umbilical cord. The membranes (afterbirth) must be carefully inspected to be sure everything has been expelled (fig. 28.21c). If any of these structures remain in the uterus, they can cause post-partum hemorrhaging. The umbilical blood vessels are counted because an abnormal number in the cord may indicate cardiovascular abnormalities in the infant.

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