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For the remainder of the pregnancy, placental estrogens and placental progesterone maintain the uterine wall. The placenta also secretes a hormone called pla-cental lactogen that may stimulate breast development and prepare the mammary glands for milk secretion, with the aid of placental estrogens and progesterone. Pla-cental progesterone and a polypeptide hormone called relaxin from the corpus luteum inhibit the smooth mus


Relative concentrations of hCG, estrogens, and progesterone in the blood during pregnancy.

cles in the myometrium, suppressing uterine contractions until the birth process begins.

The high concentration of placental estrogens during pregnancy enlarges the vagina and the external reproductive organs. Also, relaxin relaxes the ligaments holding the symphysis pubis and sacroiliac joints together. This action, which usually occurs during the last week of pregnancy, allows for greater movement at these joints, aiding passage of the fetus through the birth canal.

Other hormonal changes that occur during pregnancy include increased secretions of aldosterone from the adrenal cortex and of parathyroid hormone from the parathyroid glands. Aldosterone promotes renal reabsorption of sodium, leading to fluid retention. Parathyroid hormone helps to maintain a high concentration of maternal blood calcium, since fetal demand for calcium can cause hypocalcemia, which promotes cramps. Table 22.5 summarizes the hormonal changes of pregnancy.

C>0 Reconnect to chapter 13, Parathyroid Glands, page 523.

99 What mechanism maintains the uterine wall during pregnancy?

^9 What is the source of hCG during the first few months of pregnancy?

^9 What is the source of the hormones that sustain the uterine wall during pregnancy?

□ What other hormonal changes occur during pregnancy?

Other Changes During Pregnancy

Other changes in a woman's body respond to the increased demands of a growing fetus. As the fetus grows,

1. Following implantation, cells of the trophoblast begin to secrete hCG.

1. Following implantation, cells of the trophoblast begin to secrete hCG.

2. hCG maintains the corpus luteum, which continues secreting estrogens and progesterone.

3. As the placenta develops, it secretes large quantities of estrogens and progesterone.

4. Placental estrogens and progesterone a. stimulate the uterine lining to continue development.

b. maintain the uterine lining.

c. inhibit secretion of FSH and LH from the anterior pituitary gland.

d. stimulate development of the mammary glands.

e. inhibit uterine contractions (progesterone).

f. enlarge the reproductive organs (estrogens).

5. Relaxin from the corpus luteum also inhibits uterine contractions and relaxes the pelvic ligaments.

6. The placenta secretes placental lactogen that stimulates breast development.

7. Aldosterone from the adrenal cortex promotes reabsorption of sodium.

8. Parathyroid hormone from the parathyroid glands helps maintain a high concentration of maternal blood calcium.

the uterus enlarges greatly, and instead of being confined to its normal location in the pelvic cavity, it extends upward and may eventually reach the level of the ribs. The abdominal organs are displaced upward and compressed against the diaphragm. The enlarging uterus also presses on the urinary bladder.

A pregnant woman is well aware of the effects of her expanding uterus. She can no longer eat large meals, develops heartburn often as stomach contents are pushed up into the esophagus, and frequently has to urinate as her uterus presses on her bladder.

As the placenta grows and develops, it requires more blood, and as the fetus enlarges, it needs more oxygen and produces more waste that must be excreted. The pregnant woman's blood volume, cardiac output, breathing rate, and urine production all increase to handle fetal growth.

The pregnant woman must eat more to obtain adequate nutrition for the fetus. Her intake must supply sufficient vitamins, minerals, and proteins for herself and the fetus. The fetal tissues have a greater capacity to capture available nutrients than do the maternal tissues. Consequently, if the pregnant woman's diet is inadequate, her body will usually show symptoms of a deficiency condition before fetal growth is adversely affected.

Fetus is moved downward

Reflex is elicited that causes stronger uterine contractions

Figure 22.39

The birth process involves this positive feedback mechanism.

Fetus is moved downward

Reflex is elicited that causes stronger uterine contractions

Figure 22.39

The birth process involves this positive feedback mechanism.


Pregnancy usually continues for forty weeks, or about nine calendar months, if it is measured from the beginning of the last menstrual cycle. Pregnancy terminates with the birth process (parturition).

Birth is a complex, little-understood process. Progesterone plays a major role in its start. During pregnancy, this hormone suppresses uterine contractions. As the placenta ages, the concentration of progesterone within the uterus declines, which may also stimulate synthesis of a prostaglandin that promotes uterine contractions.

Stretching of the uterine and vaginal tissues late in pregnancy also stimulates the birth process. This may initiate nerve impulses to the hypothalamus, which, in turn, signals the posterior pituitary gland to release the hormone oxytocin, which stimulates powerful uterine contractions. Combined with the greater excitability of the myometrium due to the decline in progesterone secretion, oxytocin aids labor in its later stages.

During labor, muscular contractions force the fetus through the birth canal. Rhythmic contractions that begin at the top of the uterus and travel down its length force the contents of the uterus toward the cervix.

Since the fetus is usually positioned head downward, labor contractions force the head against the cervix. This action stretches the cervix, which elicits a reflex that stimulates still stronger labor contractions. Thus, a positive feedback system operates in which uterine contractions produce more intense uterine contractions until a maximum effort is achieved (fig. 22.39). At the same time, dilation of the cervix reflexly stimulates an increased release of oxytocin from the posterior pituitary gland.

Factors Contributing to the Labor Process

1. As the time of birth approaches, secretion of progesterone declines, and its inhibiting effect on uterine contractions may lessen.

2. Decreasing progesterone concentration may stimulate synthesis of prostaglandins, which may initiate labor.

3. Stretching uterine tissues stimulates release of oxytocin from the posterior pituitary gland.

4. Oxytocin may stimulate uterine contractions and aid labor in its later stages.

5. As the fetal head stretches the cervix, a positive feedback mechanism results in stronger and stronger uterine contractions and a greater release of oxytocin.

6. Positive feedback stimulates abdominal wall muscles to contract with greater and greater force.

7. The fetus is forced through the birth canal to the outside.

An infant passing through the birth canal can stretch and tear the tissues between the vulva and anus (perineum). Before the birth is complete, a physician may make an incision along the midline of the perineum from the vestibule to within 1.5 centimeters of the anus. This procedure, called an episiotomy, ensures that the perineal tissues are cut cleanly rather than torn, which aids healing.

As labor continues, abdominal wall muscles are stimulated to contract. These muscles also help move the fetus through the cervix and vagina to the outside. Table 22.6 summarizes some of the factors promoting labor. Figure 22.40 illustrates the steps of the birth process.

Following birth of the fetus, the placenta, which remains inside the uterus, separates from the uterine wall and is expelled by uterine contractions through the birth canal. This expulsion, termed the afterbirth, is accompanied by bleeding, because vascular tissues are damaged in the process. However, the loss of blood is usually minimized by continued contraction of the uterus that compresses the bleeding vessels. The action of oxytocin stimulates this contraction.

For several weeks following childbirth, the uterus shrinks by a process called involution. Also, its en-dometrium sloughs off and is discharged through the vagina. The new mother passes a bloody and then yellowish discharge from the vagina for a few weeks. This is followed by the return of an epithelial lining characteristic of a nonpregnant female. Clinical Application 22.4 addresses some causes of infertility in the female.

H List some of the physiological changes that occur in a woman's body during pregnancy.

Describe the role of progesterone in initiating labor.

Explain how dilation of the cervix affects labor.

Explain how bleeding is controlled naturally after the placenta is expelled.

Mammary Glands

The mammary glands are accessory organs of the female reproductive system that are specialized to secrete milk following pregnancy.

Location of the Glands

The mammary glands are located in the subcutaneous tissue of the anterior thorax within the hemispherical elevations called breasts. The breasts overlie the pectoralis major muscles and extend from the second to the sixth ribs and from the sternum to the axillae.

A nipple is located near the tip of each breast at about the level of the fourth intercostal space. It is surrounded by a circular area of pigmented skin called the areola (fig. 22.41).

Structure of the Glands

A mammary gland is composed of fifteen to twenty irregularly shaped lobes. Each lobe contains glands (alveolar glands) and a duct (lactiferous duct) that leads to the nipple and opens to the outside. Dense connective and adipose tissues separate the lobes. These tissues also support the glands and attach them to the fascia of the underlying pectoral muscles. Other connective tissue, which forms dense strands called suspensory ligaments, extends inward from the dermis of the breast to the fascia, helping support the breast's weight. Clinical Application 22.5 discusses breast cancer.

Development of the Breasts

The mammary glands of boys and girls are similar. As children reach puberty, the glands in males do not develop, whereas ovarian hormones stimulate development of the glands in females. As a result, the alveolar glands and ducts enlarge, and fat is deposited so that each breast becomes surrounded by adipose tissue, except for the region of the areola.

During pregnancy, placental estrogens and progesterone stimulate further development of the mammary glands. Estrogens cause the ductile systems to grow and branch, and deposit abundant fat around them. Progesterone stimulates the development of the alveolar glands at the ends of the ducts. Placental lactogen also promotes these changes.

Because of hormonal activity, the breasts may double in size during pregnancy. At the same time, glandular tissue replaces the adipose tissue of the breasts. Beginning about the fifth week of pregnancy, the anterior


Symphysis pubis /Urinary bladder

Symphysis pubis /Urinary bladder


Vagina Cervix Rectum

Amniotic sac

Ruptured amniotic sac

Ruptured amniotic sac



Figure 22.40

Stages in birth. (a) Fetal position before labor, (b) dilation of the cervix, (c) expulsion of the fetus, (d) expulsion of the placenta.





Figure 22.40

Stages in birth. (a) Fetal position before labor, (b) dilation of the cervix, (c) expulsion of the fetus, (d) expulsion of the placenta.

pituitary gland releases increasing amounts of prolactin. Prolactin is synthesized from early pregnancy throughout gestation, peaking at the time of birth. However, milk secretion does not begin until after birth. This is because during pregnancy, placental progesterone inhibits milk production, and placental lactogen blocks the action of prolactin. Consequently, even though the mammary glands can secrete milk, none is produced. The micrographs in figure 22.42 compare the mammary gland tissues of a nonpregnant woman with those of a lactating woman.

Milk Production and Secretion

Following childbirth and the expulsion of the placenta, the maternal blood concentrations of placental hormones decline rapidly. The action of prolactin is no longer inhibited.

Prolactin stimulates the mammary glands to secrete large quantities of milk. This hormonal effect does not occur until two or three days following birth, and in the meantime, the glands secrete a thin, watery fluid called colostrum. Although colostrum is rich in proteins, partic ularly protective antibodies, its concentrations of carbohydrates and fats are lower than those of milk.

Milk does not flow readily through the ductile system of the mammary gland, but must be actively ejected by contraction of specialized myoepithelial cells surrounding the alveolar glands. A reflex action controls this process and is elicited when the breast is suckled or the nipple or areola is otherwise mechanically stimulated (fig. 22.43). Then, impulses from sensory receptors within the breasts travel to the hypothalamus, which signals the posterior pituitary gland to release oxytocin. The oxytocin reaches the breasts by means of the blood and stimulates the myoepithelial cells to contract (in both breasts). Within about thirty seconds, milk is ejected into a suckling infant's mouth (fig. 22.44).

Sensory impulses triggered by mechanical stimulation of the nipples also signal the hypothalamus to continue secreting prolactin. Thus, prolactin is released as long as milk leaves the breasts. However, if milk is not removed regularly, the hypothalamus inhibits the secretion of prolactin, and within about one week, the mammary glands lose their capacity to produce milk.

Female Infertility

For one out of six couples, trying for parenthood is a time of increasing concern, as pregnancy remains elusive. Physicians define infertility as the inability to conceive after a year of trying. A physical cause is found in 90% of cases, and 60% of the time, the abnormality lies in the female's reproductive system.

One of the more common causes of female infertility is hypose-cretion of gonadotropic hormones from the anterior pituitary gland, followed by failure to ovulate (anovulation). This type of anovulatory cycle can sometimes be detected by testing the female's urine for pregnane-diol, a product of progesterone metabolism. Since the concentration of progesterone normally rises following ovulation, no increase in pregnanediol in the urine during the latter part of the menstrual cycle suggests lack of ovulation.

Fertility specialists can treat absence of ovulation due to too little secretion of gonadotropic hormones by administering hCG (obtained from human placentas) or another ovulation-stimulating biochemical, human menopausal gonadotropin (hMG), which contains LH and FSH and is obtained from urine of women who are past menopause. However, either hCG or hMG may overstimulate the ovaries and cause many follicles to release egg cells simultaneously, resulting in multiple births if fertilization occurs.

Another cause of female infertility is endometriosis, in which tissue resembling the inner lining of the uterus (endometrium) grows in the abdominal cavity. This may happen if small pieces of the endometrium move up through the uterine tubes during menses and implant in the abdominal cavity. Here the tissue changes as it would in the uterine lining during the menstrual cycle. However, when the tissue begins to break down at the end of the cycle, it cannot be expelled to the outside. Instead, material remains in the abdominal cavity where it may irritate the lining (peritoneum) and cause con siderable abdominal pain. These breakdown products also stimulate formation of fibrous tissue (fibrosis), which may encase the ovary and prevent ovulation or obstruct the uterine tubes. Conception becomes impossible.

Some women become infertile as a result of infections, such as gonorrhea. Infections can inflame and obstruct the uterine tubes or stimulate production of viscous mucus that can plug the cervix and prevent entry of sperm.

The first step in finding the right treatment for a particular patient is to determine the cause of the infertility. Table 22D describes diagnostic tests that a woman who is having difficulty conceiving may undergo. ■


What It Checks

Hormone levels

If ovulation occurs


Placement and appearance of reproductive organs and structures

Postcoital test

Cervix examined soon after unprotected intercourse to see if mucus is thin enough to allow sperm through

Endometrial biopsy

Small piece of uterine lining sampled and viewed under microscope to see if it can support an embryo


Dye injected into uterine tube and followed with scanner shows if tube is clear or blocked


Small, lit optical device inserted near navel to detect scar tissue blocking tubes, which ultrasound may miss


Scar tissue in tubes removed through incision made for laparoscopy

(Lewis, Human Genetics Table 20.1)

(Lewis, Human Genetics Table 20.1)

Alveolar duct (b)

Figure 22.41

Structure of the breast. (a) Sagittal section, (fa) Anterior view.

Alveolar duct (b)

Glandular tissue

Connective tissue

Glandular tissue

Connective tissue

Mammal Lymphatic Vessel Microscope
Glandular tissue with secretions

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