Anatomic and physiologic changes of normal pregnancy

Keywords; Pregnancy, anatomic changes, physiologic changes, development in pregnant women, embryological structures

Changes in organs and tissues directly associated with childbearing

Ovaries

The ovaries of a nonpregnant young woman who is in good health go through cyclic changes each month. These changes centre about a follicle, or “egg sac.” A new follicle develops after each menstrual period, casts off an egg (ovulation), and, after ovulation, forms a new structure (the corpus luteum).

If the egg is fertilized, it is sustained for a short time by the hormones produced by the corpus luteum. Progesterone and estrogen, secreted by the corpus luteum, are essential for the preservation of the pregnancy during its early months. If pregnancy does not occur, the egg disintegrates and the corpus luteum shrinks. As it shrinks, the stimulating effect of its hormones, progesterone and estrogen, is withdrawn from the endometrium (the lining of the uterus), and menstruation occurs. The cycle then begins again.

Pregnancy, if it occurs, maintains the corpus luteum by means of the hormones produced by the young placenta. The corpus luteum is not essential in human pregnancy after the first few weeks because of the takeover of its functions by the placenta. In fact, human pregnancies have gone on undisturbed when the corpus luteum has been removed as early as the 41st day after conception. Gradually the placenta, or afterbirth, begins to elaborate progesterone and estrogen itself. By the 70th day of pregnancy the placenta is unquestionably able to replace the corpus luteum without endangering the pregnancy during the transfer of function. At the end of pregnancy the corpus luteum has usually regressed until it is no longer a prominent feature of the ovary.

During the first few months of pregnancy the ovary that contains the functioning corpus luteum is considerably larger than the other ovary. During pregnancy, both ovaries usually are studded with fluid-filled egg sacs as a result of chorionic gonadotropin stimulation; by the end of pregnancy, most of these follicles have gradually regressed and disappeared.

The blood supply to both ovaries is increased during pregnancy. Both glands frequently reveal plaques of bright red fleshy material on their surfaces, which, if examined microscopically, demonstrate the typical cellular change of pregnancy, called a decidual reaction. In this reaction, cells develop that look like the cells in the lining of the pregnant uterus. They result from the high hormone levels that occur during pregnancy and disappear after the pregnancy terminates.

The uterus and the development of the placenta

The uterus is a thick-walled, pear-shaped organ measuring seven centimetres (about 2.75 inches) in length and weighing 30 grams (about one ounce) in an unpregnant woman in her later teens. It has a buttonlike lower end, the cervix, that merges with the bulbous larger portion, called the corpus. The corpus comprises approximately three-fourths of the uterus. There is a flat, triangular-shaped cavity within the uterus. At term, the uterus is a large, thin-walled, hollow, elastic, fluid-filled cylinder measuring approximately 30 centimetres (about 12 inches) in length, weighing approximately 1,200 grams (2.6 pounds), and having a capacity of 4,000 to 5,000 millilitres (4.2 to 5.3 quarts).

The greater size of the uterus as a result of pregnancy is due to a marked increase in the number of muscle fibres, blood vessels, nerves, and lymphatic vessels in the uterine wall. There is also a five- to tenfold increase in the size of the individual muscle fibre and marked enlargement in the diameters of the blood and lymph vessels.

During the first few weeks of pregnancy, the shape of the uterus is unchanged, but the organ becomes gradually softer. By the 14th week it forms a flattened or oblate spheroid. The fibrous cervix becomes remarkably softer and acquires a protective mucus plug within its cavity, but otherwise it changes little before labour. The lower part of the corpus, the isthmus, first becomes elongated and then, as the uterine contents demand more space, stretches and unfolds to form a bowl-shaped formation called the lower uterine segment. The fibrous nature of the cervix causes it to resist this unfolding action.

The uterine wall is stretched and thinned during pregnancy by the growing conceptus, as the whole product of conception is called, and by the fluid that surrounds it. By term, this process converts the uterus into an elastic, fluid-filled cylinder. It is only late in pregnancy that the cervix gradually thins out and softens; during labour it dilates for passage of the infant.

As pregnancy progresses, the uterus rises out of the pelvis and fills the abdominal cavity. It is top-heavy near term so that it falls forward and, because of the large bowel on the left side, rotates to the right. It presses on the diaphragm and pushes the other organs aside. The uterus may sink downward in the pelvis several weeks before term in a process that is known as lightening or dropping. This occurs as the fetal head descends into the pelvis. In some women, particularly those who have borne children, lightening does not occur until the onset of labour. Lightening may be impossible in women who have an abnormally small pelvis, an oversized fetus, or a fetus lying in an abnormal position.

For a short time after fertilization, the conceptus, a minute bubblelike structure called a blastocyst, lies unattached in the uterine cavity. The cells that will become the embryo (the embryonic disk) form a thickened layer on one side of the bubble. Elsewhere, the walls of the bubble consist of a single layer of cells; these cells are the trophoblast, which has a special ability to attach to and invade the uterine wall. The trophoblast plays an important role later in the development of the placenta or afterbirth. The conceptus makes contact with the uterine lining about the fifth or sixth day after conception. After contact the blastocyst collapses to form a rounded disk with the embryonic mass on the surface and the trophoblast against the endometrium (uterine lining). The part of the trophoblast that is in contact with the endometrium grows into and invades the maternal tissue. Concomitant disintegration of the endometrium allows the conceptus to sink into the uterine lining.

Soon the entire blastocyst is buried in the endometrium. Proliferation of the trophoblast over the part of the collapsed bubble that is opposite the embryo is part of the implantation procedure that helps to cover the blastocyst. After a few days, a cavity forms that bears the same relation to the embryonic disk that the blastocyst cavity did before; this cavity will become the fluid-filled chorionic cavity containing the embryo. Ultimately it will contain the amniotic fluid that surrounds the fetus, the fetus itself, and the umbilical cord.

The body stalk, which will become the umbilical cord, then begins to separate the embryo from the syncytiotrophoblast, the outer layer of the trophoblast lying against the endometrium; the inner lining of the trophoblast is called cytotrophoblast. As the syncytiotrophoblast advances into the endometrium, it surrounds minute branches of the uterine arteries that contain maternal blood. Erosion of the endometrium about these blood sinuses allows them to open into the small cavities in the trophoblast. The cytotrophoblast, which lines the cavity, forms fingers of proliferating cells extending into the syncytiotrophoblast. After the placenta is developed, these fingers will be the cores of the root-like placental villi, structures that will draw nutrients and oxygen from the maternal blood that bathes them. This is the first step in uteroplacental circulation, which supplies the fetus with all of the sustenance necessary for life and growth and removes waste products from it. During the third week of pregnancy, the syncytiotrophoblast forms a single layer of cells covering the growing villi and lining the syncytial lacunae or small cavities between the villi. The conceptus is buried in the endometrium, and its whole surface is covered at this time by developing villi. The greater part of the chorionic wall is now cytotrophoblast. Fingers of cytotrophoblast in the form of cell masses extend into the syncytial layer. Soon thereafter, a layer of connective tissue, or mesoderm, grows into the villi, which now form branches as they spread out into the blood-filled spaces in the endometrium adjacent to the conceptus.

By the end of the third week, the chorionic villi that form the outer surface of the chorionic sac are covered by a thick layer of cytotrophoblast and have a connective tissue core within which embryonic blood vessels are beginning to develop. The vessels, which arise from the yolk sac, connect with the primitive vascular system in the embryo. As growth progresses the layer of cytotrophoblast begins to regress. It disappears by the fifth month of pregnancy.

The layer of endometrium closest to the encroaching conceptus forms, with remnants of the invading syncytio-trophoblast, a thin plate of cells known as the decidua basalis, the maternal component of the mature placenta; it is cast off when the placenta is expelled. The fetal part of the placenta—the villi and their contained blood vessels—is separated from the decidua basalis by a lakelike body of fluid blood. This pool was created by coalescence of the intervillous spaces. The intervillous spaces in turn were formed from the syncytial lacunae in the young conceptus. Maternal blood enters this blood mass from the branches of the uterine arteries. The pool is drained by the uterine veins. It is so choked by intermingling villi and their branches that its continuity is lost on gross inspection.

The chorionic cavity contains the fluid in which the embryo floats. As its shell or outer surface becomes larger, the decidua capsularis, which is that part of the endometrium that has grown over the side of the conceptus away from the embryo (i.e., the abembryonic side) after implantation, becomes thinner. After 12 weeks or so, the villi on this side, which is the side directed toward the uterine cavity, disappear, leaving the smooth chorion, now called the chorion laeve. The chorion frondosum is that part of the conceptus that forms as the villi grow larger on the side of the chorionic shell next to the uterine wall. The discus-shaped placenta develops from the chorion frondosum and the decidua basalis.

At term, the normal placenta is a disk-shaped structure approximately 16 to 20 centimetres (about six to seven inches) in diameter, three or four centimetres (about 1.2–1.6 inches) in thickness at its thickest part, and weighing between 500 and 1,000 grams (1.1 and 2.2 pounds). It is thinner at its margins, where it is joined to the membrane-like chorion which spreads out over the whole inner surface of the uterus and contains the fetus and the amniotic fluid. The amnion, a thinner membrane, is adherent to and covers the inner surface of the chorion. The inner or fetal surface of the placenta is shiny, smooth, and traversed by a number of branching fetal blood vessels that come together at the point—usually the centre of the placenta—where the umbilical cord attaches. The maternal or uterine side of the placenta, covered by the thin, flaky decidua basalis, a cast-off part of the uterine lining, is rough and purplish-red, and has a raw appearance. When the placenta is cut across, its interior is seen to be made of a soft, crepelike or spongy matrix from which semisolid or clotted blood, caught when it is separated from the uterine wall to which it was attached, can be squeezed. Detailed examination shows that the villi and their branches form an arborescent (treelike) mass within the huge blood lake of the intervillous space. Anchoring villi extend outward from the fetal side and fuse with the decidua basalis to hold the organ's shape. Others, algae-like, float freely in the blood lake. Dividing partitions, formed from the trophoblast shell, project into the intervillous space from the decidual side. They divide the placenta into 15 or 20 compartments, which are called cotyledons.

Maternal blood flows from the uterine vessels into the trophoblast-lined intervillous blood lake. Within each villus is a blood vessel network that is part of the fetal circulatory system. Blood within the villous vessel is circulated by the fetal heart. The blood vessel wall, the connective tissue of the villous core, and the syncytiotrophoblast covering the villus lie between the fetal and the maternal bloodstreams. This is known as the placental barrier. As pregnancy progresses, the fetal blood vessels become larger, the connective tissue stretches over them, and the syncytiotrophoblastic layer becomes fragmentary. As a result, the placental barrier becomes much thinner. Normally, blood cells and bacteria do not pass through it, but nutrients, water, salt, viruses, hormones, and many other substances, including many drugs, can filter across it.

Uterine tubes

One of the two uterine tubes is the pathway down which the ripe ovum travels on its way to the uterus or womb. The spermatozoa from the male migrate up the tube, and it is there that they meet the ovum and fertilization occurs. During the first few days after fertilization the zygote, or fertilized egg, moves downward in the tube toward the uterus. While it is lying free in the tubal canal, the young conceptus is nourished by secretions from the tube. After the fertilized egg (or conceptus) passes into the uterus, the tube ceases to play any part in the pregnancy; in fact, the only function the tube has is carried out during those few days before, during, and after conception. As pregnancy goes on, the tube gradually enlarges, however, and contains more blood, as do all the pelvic organs; some of its cells may show a reaction, called a decidual reaction, to the hormones of pregnancy. As the uterus increases in size, the tubes stretch upward with it until they become two greatly enlarged elongated strands, one on each side of the uterus.

Vagina

The pinkish tan colour of the lining of the vagina gradually takes on a bluish cast during the early months of pregnancy as a result of the dilation of the blood vessels in the vaginal wall; later the vaginal wall tends to become a purplish red colour as the blood vessels become further engorged. The cells of the vaginal mucosa increase in size. Added numbers of these cells peel off the surface of the mucosa and mix with the increased vaginal fluid. This produces a profuse vaginal secretion. Thickening, softening, and relaxation of the loosely folded, succulent lining of the vagina and the sodden tissues beneath it greatly increase distensibility and capacity of the vaginal cavity; this is a process that partially prepares the birth canal for the passage through it of the large fetal mass.

External genital structures

Changes in the external genitalia are similar to those in the vagina. The tissues become first softened and more succulent and later extremely fragile, as an increasing amount of blood and fluid collects in them. They take on a purplish red colour because of increased blood supply. Darkening of the vulvar skin, frequently seen during pregnancy, is particularly common among women of Mediterranean ethnic groups.

Other pelvic tissues

The pelvic blood vessels and lymph channels become larger and longer. They develop new branches adequate to transport the greatly increased amounts of blood and tissue fluid that accumulate in the uterus and the other pelvic organs during pregnancy. Congestion and engorgement of blood in the pelvis, both within and without the uterus, are characteristic of pregnancy.

Changes in the muscles, ligaments, and other supporting tissues of the pelvis begin early in pregnancy and become progressively more pronounced as pregnancy continues. These changes are induced by the greatly increased hormonal levels in the mother's blood that characterize pregnancy. Before labour starts, the pelvic supporting tissues must have sufficient elasticity and strength to permit the uterus to grow out of the pelvis and yet support it. The muscles must be soft and elastic enough during delivery so that they can stretch apart and not obstruct the baby's birth. Softening and greater elasticity is brought about not only by the growth of new tissue but also by congestion and retained fluid within the tissues themselves.

The bones forming the mother's pelvis show relatively few changes during pregnancy. Loosening of the joint between the pubic bones in front and of the joints between the sacrum and the pelvis in back occurs as a response to the hormone called relaxin, which is produced by the ovary. Although relaxin, which causes marked separation of the pelvic joints in some animals, usually has too slight an effect in human beings to be noticed, softening of the attachments between the bones may be sufficient to cause a few women considerable distress. The strain on the joint between the sacrum and the spine becomes greater near term when the woman tilts her pelvis forward and bends the upper part of her body backward to compensate for the weight of the heavy uterus. When relaxation is excessive, the woman suffers from backache and difficulty in walking. If it is extreme, she may have a waddling gait. Relaxation of the pelvic joints does not disappear quickly after delivery; it accounts for much of the backache that women with new babies experience.

The mother's bones show no structural change if her calcium reserve and intake are normal. If her reserve and intake are not adequate, the fetus may draw so much calcium from her bones that the bones become soft and deformed. This condition is rarely seen, except in areas of the world where extreme poverty and serious calcium deficiency are major problems.

Breasts

The earliest changes in the breasts during pregnancy are an exaggeration of the frequently experienced premenstrual discomfort and fullness. The sensation is so specific for pregnancy that many women who have been pregnant before are made aware of their condition by the feeling that they have in their breasts. As pregnancy progresses the breasts become larger, the lightly pigmented area (areola) around each nipple becomes first florid or dusky in colour and then appreciably darker; during the later months the areola takes on a hue that is deep bronze or brownish black, depending on the woman's natural pigmentation. The veins beneath the skin over the breast become enlarged and more prominent. The small oily or sebaceous glands (glands of Montgomery) about the nipple become prominent.

These changes are due to the greatly increased levels of estrogen and progesterone in the woman's blood. These ovarian hormones also prepare the breast tissue for the action of the lactogenic (milk-causing) hormone, prolactin, produced by the pituitary gland. During the later part of pregnancy a milky fluid, colostrum, exudes from the ducts or can be expressed from them.

After delivery the decrease in estrogen and progesterone levels presumably permits the pituitary gland to release prolactin, which causes the breast to secrete milk. It is thought that the high hormonal levels inhibit the action or secretion of prolactin before delivery. Prolactin continues to be produced, and lactation usually continues, as long as the mother feeds her baby at the breast.

Anatomic and physiologic changes in other organs and tissues

Cardiovascular and lymphatic systems

During pregnancy the increasing needs of the growing fetus and of her own tissues throw an added burden on the mother's heart. The work that the heart does is measured by the amount of blood it expels per minute (the cardiac output). Rapid increase in the cardiac output occurs between the 9th and the 14th week of gestation. During the period from the 28th to the 30th week, when the load is heaviest, the heart of a pregnant woman is doing 25 to 30 percent more work than it was doing before pregnancy. As the time of delivery approaches, the heart's workload diminishes to some extent; when the baby is born, the load is approximately equal to what it was when the mother was in the nonpregnant state. This decrease in cardiac output and cardiac work, which occurs in spite of the continued needs of the fetus and of the maternal tissues for blood-borne oxygen and nutriments, is explained by the more efficient way that the tissues draw on the mother's blood for oxygen and nourishment during the terminal weeks of pregnancy.

The position of the heart is changed to a greater or lesser degree during pregnancy. As the uterus enlarges, it elevates the diaphragm. This in turn pushes the heart upward, to the left, and somewhat forward, so that it is nearer the chest wall beneath the breast. Near the end of gestation the large uterus may raise the heart until the latter lies almost at a right angle to the long axis of the woman's body. These changes, which also bring some rotation of the heart, vary considerably in different individuals. When present to a marked degree, they may give an examining physician the erroneous impression that a normal heart is considerably enlarged. Actually, in spite of its greater workload, a healthy heart enlarges little or not at all even during the midportion of pregnancy, when the load is greatest.

Changes in the position of the heart, the greater workload, the increased volume of blood that the heart expels per beat, the decreased viscosity of the blood, and the larger amount of blood in the woman's blood vessels (discussed below) will, in many women, cause some distortion of the sounds that the physician hears when listening to a patient's heart with a stethoscope. Such distorted sounds, called “functional” murmurs (as distinguished from “organic” murmurs, which may be present when the heart is diseased), do not indicate that anything is amiss, although they may be sufficiently atypical to cause the obstetrician to refer the patient to a cardiologist for evaluation. Pregnancy sometimes produces minor changes in the electrocardiogram, but these changes are within normal limits.

Such is the ability of the heart to respond to an increased workload that even the pregnant woman with serious heart disease, given proper care and without an unexpected complication, will usually go through her pregnancy and delivery without a catastrophe. She may, however, encounter difficulty when she tries to cope with the stress of caring for her family after the baby is born.

Normal pregnancy does not increase the mother's blood pressure. Indeed, a slight lowering of the blood pressure is commonly noted during the course of the pregnancy. Any notable rise in a pregnant woman's blood pressure is reason for alertness on the part of her physician, and, if it continues to rise, for concern; it usually foretells the onset of preeclampsia (see below).

The pulse rate is a trifle more rapid during pregnancy, reflecting the more rapid heartbeat that is necessary in order to move the larger volume of blood present. The rate at which blood flows through the myriad of small blood vessels in the skin (the peripheral circulation) is accelerated during pregnancy, leading to the elevated skin temperature, the tendency to perspire, and, in part, to the redness of the palms and the tiny dilated blood vessels in some women as their pregnancies progress.

The most notable change in the circulatory system during pregnancy, other than those described in the heart, is a slowing of the blood flow in the lower extremities. With this decrease in the rate of flow there is an increase in the pressure within the veins and some stasis—stagnation—of the blood in the legs. These changes, which are believed to be caused primarily by the pressure of the uterus on the large blood vessels in the pelvis, are progressive during pregnancy and disappear after delivery. They also are thought to be caused in part by the marked increase in the amounts of the hormones estrogen and progesterone in the circulating blood. Increased venous pressure, slowing of the rate of venous flow, and partial stasis of the blood in the veins are major factors in causing the swelling of the legs and the varicose (abnormally dilated) veins of the lower legs that are commonly present near the end of pregnancy.

The lymphatic vessels of the pregnant woman's pelvis become enlarged in response to the increased amount of tissue fluid in the engorged pelvic organs. As the uterus grows in size, it presses on these channels, causing impairment of the lymphatic drainage from the woman's legs, with resultant swelling and distention of her feet and legs.

Although some fluid almost invariably collects in the feet, ankles, and legs near the time of delivery, sudden swelling of the feet and legs or a notable increase in swelling may be an early signal of impending preeclampsia, a serious disorder of pregnancy that is discussed below. Generalized swelling—i.e., swelling of the hands, face, and other parts of the body—is a cause for serious concern.

Respiratory tract

One would expect that, as the uterus grows larger and pushes the diaphragm up, it would interfere with breathing, but the lungs actually work as efficiently as they do in the nonpregnant state. This is due to a change in the shape of the chest cavity during pregnancy; the chest diameter increases as its height decreases, so that there is actually a slight increase in the space that the lungs occupy.

The amount of air drawn in and expelled per minute by the lungs increases progressively during pregnancy. Immediately before delivery the number of breaths per minute is approximately twice what it is after the baby is born. This, like so many of the other changes in the mother's body, is an adaptation of one of her vital functions that is necessary to supply her tissues and those of the growing fetus with increasing amounts of oxygen.

Gastrointestinal tract

A number of alterations, often causing more or less distress, occur in the physical condition and functions of the gastrointestinal tract during pregnancy.

Disturbances of the sensations of taste and smell, relatively common during early months of gestation, are often accompanied by a dislike of odours and a distaste of foods formerly found to be agreeable. The inflammation of the mouth and gums that some pregnant women complain of is more often caused by poor oral hygiene, by vitamin deficiencies, or by anemia than by the pregnancy itself.

Hydrochloric acid and pepsin, adequate amounts of which are necessary for satisfactory digestion, are produced by the stomach in decreased amounts during pregnancy. This decrease in the amount of acid in the stomach may explain some of the otherwise inexplicable anemias that occasionally occur during the course of an otherwise seemingly normal pregnancy.

During pregnancy the stomach muscles lose some of their tone and become more flabby, and the contractility of the stomach is reduced. As a result, the time it takes for the stomach to empty its contents into the intestinal tract is prolonged. As pregnancy progresses, the stomach is pushed upward; near term it lies like a flabby pouch across the top of the uterus instead of hanging downward, as it normally does, in a semivertical position. The loss of tone of the stomach muscles, the decrease in stomach acidity, and the change in position of the stomach are conducive to the flow of intestinal contents back into the stomach.

These disturbances in gastric function are responsible, in part at least, for the intolerance for fatty foods, the indigestion, the discomfort felt in the upper part of the abdomen, and the heartburn experienced by most pregnant women at some time during their pregnancies.

The musculature not only of the stomach but also of the entire intestinal tract loses much of its tonicity. As a result, peristalsis, the series of wavelike movements of the intestines, is slowed, the length of time it takes food to pass through the intestinal tract is prolonged, and there is more or less stagnation of the intestinal contents.

Constipation and hemorrhoids that cause rectal pain and bleeding are common complaints during pregnancy. The constipation is caused by lack of tone of the intestinal tract and stagnation of the bowel contents. Pregnant women may also lose the urge to defecate because of the pressure of the uterus on the lower bowel and inhibition of a reflex stimulus, known as the gastrocolic reflex, from the stomach to the rectum. The latter mechanism, which depends on normal stomach function, is responsible for the increased activity of the lower bowel that follows increased stomach activity, such as that induced by eating. It is this reflex that causes many persons to feel a desire to defecate within an hour or so after eating a full meal. Hemorrhoids—greatly enlarged or varicose veins in the lower rectum—that appear during pregnancy are due to constipation, to stasis of blood in the pelvic veins, and to pressure by the enlarging uterus on the blood vessels in the pelvis.

Liver

The liver, which plays an essential role in many of the vital processes—processes as diverse as participating in the metabolism of nutriments and vitamins and the elimination of the waste products of metabolism—changes anatomically and functionally during pregnancy to meet the added load placed on it by the maternal organism, the enlarging uterus, and, to a lesser extent, the growing fetus.

The liver's ability to synthesize proteins and to supply minerals and nutriments is augmented in response to the increased requirements of the mother's tissue and the fetus. The liver adjusts to the greatly augmented amounts of hormones circulating in the mother's blood during pregnancy. It helps to dispose of or detoxify the larger amounts of waste material produced by the metabolic processes in the growing fetus, the enlarging uterus, and the mother's tissues. Furthermore, the blood vessels in the liver enlarge to accommodate the larger amount of blood in the mother's blood vessels. At the same time, the liver must compensate for the larger number of circulating red blood cells.

In response to these demands, the liver increases in size and weight, and its blood vessels become larger, but otherwise its anatomic structure changes relatively little during pregnancy.

The hormones produced by the placenta and the metabolic changes in the maternal organism, rather than the fetus, are the factors responsible not only for the increased work the liver does but also for many of the physical and functional alterations that appear during gestation.

Urinary tract

Changes that take place in the bladder and the urethra during pregnancy are attributable to relaxation of the muscles supporting these structures, to change in position, and to pressure.

The uterus lies over the bladder and presses upon it during early pregnancy. Later the uterus rises out of the pelvis. As the uterus grows larger and moves upward, the bladder is pushed forward and pulled upward. The urethra, the tube through which urine is discharged from the bladder, is stretched and distorted. As these distortions take place, the wall of the bladder becomes thickened, the blood vessels become enlarged, and fluid collects in the tissues forming the wall of the bladder. The results are swelling, stasis of blood in the blood vessels, and some mechanical inflammation of the bladder wall.

The woman is likely to urinate frequently during the early months of pregnancy when the heavy uterus presses on the bladder. Frequent urination is less common during midpregnancy, but it recurs after the baby descends into the pelvis near the time of delivery. As the bladder and urethra are pulled upward and distorted by the growing uterus, the stretched muscles that control urination are less efficient, and the woman may lose some urine involuntarily when she coughs, sneezes, or laughs; this is known as stress incontinence.

The swelling, mechanical inflammation, and stasis of blood in the blood vessels of the bladder near the end of pregnancy are conducive to bladder infection, a symptom of which is pain on urination. A microscopic examination of the urine is necessary to differentiate between the effect of pregnancy on bladder function and the symptoms caused by a bladder infection. An untreated bladder infection may lead to serious urinary tract troubles later.

Changes in the structure and function of the ureters, the two rubbery, spaghetti-like tubes that carry urine from the kidneys to the bladder, are present in 80 percent of all pregnancies. As pregnancy progresses, each ureter becomes larger, so that it lies in multiple broad curves rather than forming an almost straight line downward from the kidney. In addition, both ureters, but particularly the right one, become greatly dilated, so that the urine flows very slowly or collects in them.

The funnellike part of the kidney, called the kidney pelvis, also becomes dilated. With this dilation of the kidney pelvis and the ureters there is also a loss of tonicity or contractility in the pelvis of the kidney and the ureters. This loss of tonicity during pregnancy is similar to that mentioned in the description of the changes in the intestinal tract. Since it is the contractility of peristalsis within the ureter that propels urine downward from the kidney into the bladder, stasis of urine in the ureter is accentuated during the pregnancy. In the nonpregnant state the hydrostatic pressure in the kidney is greater than that in the bladder; during pregnancy the situation is reversed. This change of pressure further increases the stasis of urine in the ureter and kidney pelvis. As a result, bladder infections are more serious during pregnancy, because they are more likely to involve the kidney.

After delivery the ureters rapidly return to their normal condition.

The kidney of a healthy person selectively filters and secretes water, sodium, potassium, chlorides, protein, and other substances from the blood. It then reabsorbs water and essential elements in amounts that are needed to maintain the fluid, electrolytic, and other chemical balances in the body. It also filters waste products of metabolism from the blood and excretes them in the urine. During pregnancy the kidney continues to carry on these functions. The workload placed on it, however, is greater because of the increase in the amount of water and blood and in the rate of metabolism during gestation.

In early pregnancy, secretion of large amounts of dilute urine of decreased acidity, together with pressure of the uterus on the bladder, causes frequency of urination and nocturnal voiding. Less urine is excreted toward the end of pregnancy. The storage of large amounts of nitrogen, as part of the metabolism of proteins, causes a decrease in the urinary excretion of urea and of total nitrogen during gestation.

Although many healthy pregnant women occasionally show a trace of protein (albumin) in their urine, the detection of even small amounts of protein in the urine is a cause for alertness on the part of a physician, because anything more than an extremely small amount may be the first signal of impending preeclampsia or kidney disease, both of which are serious complications.

The kidney's ability to reabsorb sugar (glucose) is lower during pregnancy, and for this reason many pregnant women have transient periods during which their urine contains small amounts of glucose; such women have unimpaired ability to metabolize carbohydrates and have normal sugar levels in the blood. Glucose in the urine also may be the first sign that a person has diabetes mellitus, however; consequently, a pregnant woman whose urine contains traces of glucose is tested to make sure that she can metabolize sugar normally.

The preceding discussion of kidney function illustrates the need for a pregnant woman to be under a health-care provider's care, an essential part of which is periodic examination of her urine for protein, sugar, pus, bacteria, and other abnormal constituents.

Blood

The total amount of blood in a pregnant woman's body has increased by approximately 25 percent by the time of delivery. The increase is accounted for by the augmented volume of blood plasma (the liquid part of the blood), which is caused by fluid retention, plus an increase in the total number of red blood cells. Additional blood is needed to fill the large vessels of the uterus. Also, more blood is required to carry the oxygen and nutriments needed by the fetus and the maternal tissues and to carry away waste products. Furthermore, it is a protective reserve in case of hemorrhage during delivery.

During pregnancy the blood-forming organs, such as the bone marrow, make more erythrocytes, or red blood cells, which carry iron and oxygen. Despite this, there is usually a decrease in a pregnant woman's blood cell count—the number of red cells per cubic millimetre of blood—because the amount of blood plasma increases approximately 30 percent, while the total number of red blood cells increases by only about 20 percent. This results in apparent anemia. With these changes, the viscosity of the blood decreases and the hematocrit, which measures the relative amounts of liquid and solid constituents in the blood, is lower. Usually there is a moderate increase in the number of white blood cells per cubic millimetre during early pregnancy; this increase disappears during the latter part of pregnancy.

If a pregnant woman is otherwise healthy and receives adequate available iron for the production of hemoglobin, her red blood cell count does not ordinarily fall below 3,750,000 cells per cubic millimetre, her hemoglobin below 13.5 grams per 100 cubic millimetres of blood, and her hematocrit below 35. (Normal values for nonpregnant women are 4,200,000–5,400,000 cells, 13.8–14.2 grams hemoglobin, and 37–47 hematocrit.) Physicians usually make blood counts for their pregnant patients every two months because of the need for repeated evaluation.

Endocrine system

Most of the endocrine glands become larger, and some display alterations in function, during pregnancy; they all revert to a normal state after delivery.

The anterior lobe of the pituitary gland increases in size during pregnancy, but the production of pituitary gonadotropins, the gonad-stimulating hormones, ceases soon after the placenta begins to produce chorionic gonadotropins. The pituitary continues to secrete the hormones that stimulate the other endocrine glands. Near term, as the mother's estrogen level drops, a milk-stimulating hormone, prolactin, is produced by the pituitary. The posterior lobe of the pituitary gland does not change in size or weight during pregnancy.

The thyroid gland enlarges moderately, but there is no true increase in thyroid function during gestation. The parathyroid glands also increase in size during pregnancy but presumably are not otherwise affected by it.

The part of the pancreas that secretes insulin, the islets of Langerhans, becomes larger. Whatever increase in function is displayed may be assumed to be a balanced response to the body's demand for the products of carbohydrate metabolism. The level of plasma insulin or of insulin-like substances in the plasma is higher during pregnancy, and the destruction of insulin is also more rapid.

The blood and urinary levels of 17-hydroxycorticosteroids, hormones that affect protein, fat, and carbohydrate metabolism and that are produced by the adrenal glands, rise during pregnancy; but there is no increased effect from the hormones, because their higher level is more than offset by the increased levels of transcortin, a protein that inactivates them.

As gestation progresses, there is an elevation in the secretion of aldosterone, an adrenal hormone that plays a role in the retention of salt and water in the body. It has been suggested that this is a protective mechanism to counterbalance the tendency for progesterone to cause the excretion of sodium ions in the urine.

Skin

Pregnancy usually causes an increase in the secretion of the oil and sweat glands in the skin. Body odours may become more pronounced. Many women notice that their hair becomes thinner and drier and their nails more brittle. Others may develop an increased amount of facial and body hair. The “mask of pregnancy” seen particularly in brunettes is a deposit of brownish pigment in the skin of the forehead, the cheeks, and the nose. Puffiness and thickening of her skin may cause the pregnant woman's face to appear coarse and almost masculine. Increased pigmentation, particularly of the smooth skin about the nipples (the areolas of the breasts) and the vulva, is almost universal.

Bright red discoloration of the palms of the hands and tiny spiderweb-like red blood vessels in the skin of the arms or face are not unusual during pregnancy. Many of these changes are thought to be associated with the greatly increased levels of estrogen in the mother's bloodstream. Most of the changes disappear after delivery.

“Stretch marks,” which appear on the breasts and abdomen during pregnancy, are due to the tearing of the elastic tissues in the skin that accompanies enlargement of the breasts, distention of the abdomen, and the deposition of subcutaneous fat. They are pink or purplish red lines during pregnancy. The lines become permanent scarlike marks after delivery. Some women never develop stretch marks despite bearing several children; others lose most of the tone in their skin after one pregnancy. Stretch marks cannot be considered evidence that a woman has borne a child, however, because they sometimes are seen in women who have not been pregnant.(John W. Huffman)