So far, we have focused our attention on the develop-
mental changes in the embryo and fetus. Let’s turn our
attention to changes in the mother.
Pregnancy Changes
the Mother's Physiology
Pregnancy brings about striking changes in the mother’s
anatomy as well as her physiology. The ability of the body
to adapt to all these changes is truly remarkable. By about
the end of the third month of pregnancy, the uterus oc-
cupies most of the pelvic cavity. As the fetus continues to
grow, the uterus extends higher into the abdominal cavity.
By 38 weeks (that is, a full-term pregnancy), the fetus
and uterus occupy most of the abdominal cavity (Figure
16.16). The fetus exerts pressure on the mother’s dia-
phragm, liver, intestines, and stomach.
For some women, changes in the skin during pregnancy
may include increased pigmentation around the eyes and
cheekbones in a masklike pattern, in the areolae of the
breasts, and in the lower abdomen. Stretch marks over the
abdomen can occur as the uterus enlarges, and hair loss in-
creases. Pregnancy-induced physiological changes include
weight gain due to the fetus, amniotic fluid, the placenta,
uterine enlargement, and increased total body water; in-
creased storage of nutrients; marked breast enlargement in
preparation for lactation; and lower back pain.
Several changes occur in the mother’s cardiovascular
system. Stroke volume increases by about 30% and cardiac
output rises by 20-30% due to increased maternal blood
flow to the placenta and increased metabolism. These in-
creases are needed to meet the additional demands of the
fetus for nutrients and oxygen.
Pulmonary function is also altered during pregnan-
cy to meet the oxygen demands of the fetus. Total body
oxygen consumption can increase by 10-20%. Difficult
breathing also occurs as the expanding uterus pushes on
the diaphragm.
Pregnant women also experience an increase in ap-
petite. Pressure on the stomach may force the stomach
contents into the esophagus, resulting in heartburn. A
general decrease in gastrointestinal tract motility can
cause constipation, delay gastric emptying time, and pro-
duce nausea, vomiting, and heartburn. Pressure on the
urinary bladder by the enlarging uterus can produce uri-
nary symptoms, such as increased frequency and urgency
of urination, and stress incontinence.
Finally, changes in the reproductive system include ede-
ma and increased blood flow to the vagina. The uterus in-
creases from its nonpregnant mass of 60-80 g (2-3 ounces)
to 900-1200 g (2 pounds) at term because of increased
numbers of muscle fibers in the uterus as well as their en-
largement.
Let’s turn our attention to the hormonal regulation of
pregnancy.
Hormones Are Important for
Maintaining the Pregnancy
A number of different hormones are released during
pregnancy. The level of each of these hormones changes
as the pregnancy progresses. There appears to be a link
between some of these hormones and the development
of gestational diabetes, which results in very high blood
glucose levels in the mother (see
W h a t a H e a lth P rovider
Sees
in Chapter 9). A number of hormones are involved in
pregnancy:
Human chorionic gonadotropin
(hCG)
secreted
by the chorion maintains the corpus luteum. hCG
secretion rises and peaks by about the fourth month
of pregnancy. By this time, the placenta is established,
and the corpus luteum is no longer needed. This
hormone is the material measured by a pregnancy
test and is also the suspected cause of the morning
sickness that many women experience during the first
trimester of pregnancy.
Estrogens and progesterone are secreted initially by
the corpus luteum and later by the chorion, starting at
3 to 4 weeks, and continue to be secreted throughout
pregnancy.
Relaxin
secreted by the
corpus
luteum
and
the
placenta softens the pubic symphysis, relaxes sacroiliac
ligaments, and dilates the cervix in preparation for
labor.
Human
chorionic
somatomammotropin
(hCS),
(also called
h u m a n p la c e n ta l lactogen [h P L ])
is secreted
by the chorion; secretion increases as the placenta
grows, and peaks at about 32 weeks. hCS is thought to
help prepare the breast tissue for milk production and
lactation.
Corticotropin-releasing
hormone
(CRH)
is
secreted by the placenta starting at 12 weeks, and
secretion increases until the end of pregnancy. In
nonpregnant women, this hormone is secreted only by
the hypothalamus. CRH secretion by the placenta is
thought to be important for timing birth.
Next, we’ll look at what happens at the end of develop-
ment, during the process of labor and delivery.
496 CHAPTER 16
The Reproductive Systems
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