884
UNIT SIX
mammary glands to secrete milk, with the aid of placental
estrogens and progesterone. Placental progesterone and a
polypeptide hormone called
relaxin
from the corpus luteum
inhibit the smooth muscles 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 reproduc-
tive organs. Also, relaxin relaxes the connective tissue of the
symphysis pubis and sacroiliac joints. 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 during pregnancy include
increased secretion of aldosterone from the adrenal cortex
and of parathyroid hormone from the parathyroid glands.
Aldosterone promotes renal reabsorption of sodium, leading
to fl
uid retention. Parathyroid hormone helps to maintain a
high concentration of maternal blood calcium, because fetal
demand for calcium can cause hypocalcemia, which pro-
motes cramps.
Table 23.1
summarizes the hormonal changes
of pregnancy.
RECONNECT
To Chapter 13, Parathyroid Glands, pages 502–503.
PRACTICE
9
What are the sources of the hormones that sustain the uterine
wall during pregnancy?
10
What other hormonal changes occur during pregnancy?
Other Changes During Pregnancy
Other changes in a woman’s body respond to the increased
requirements of a growing offspring. As the offspring grows,
the uterus enlarges greatly, and instead of being conF ned 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 dia-
phragm. The enlarging uterus also presses on the urinary blad-
der. As a result, a pregnant woman may be unable to eat large
meals and may develop heartburn and have to urinate often.
The growing and developing placenta requires more
blood, and as the offspring enlarges, it needs more oxy-
gen 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 offspring
growth.
The pregnant woman must eat more to obtain adequate
nutrition for the offspring. Her intake must supply sufF cient
vitamins, minerals, and proteins for herself and the offspring.
The offspring tissues have a greater capacity to capture avail-
able nutrients than do the maternal tissues. Consequently,
if the pregnant woman’s diet is inadequate, her body will
usually show symptoms of a deF ciency condition before off-
spring growth is adversely affected.
Detecting hCG in a woman’s urine or blood is used to conF
rm preg-
nancy. The level of hCG in a pregnant woman’s body fluids peaks
at F
fty to sixty days of gestation, then falls for the remainder of her
pregnancy. Later on, measuring hCG has other uses. If a woman mis-
carries but her blood still shows hCG, fetal tissue may remain in her
uterus, and this material must be removed. At the F
fteenth week of
pregnancy, most women have a blood test that measures levels of
four substances produced by the fetus—alpha fetoprotein (A±P),
estriol (an estrogen), pregnancy-associated plasma protein A
(PAPP-A), and hCG. If estriol and PAPP-A are low but hCG is elevated,
the fetus is at risk of having an extra chromosome 21 (Down syn-
drome). ±urther tests are advisable for a deF
nitive diagnosis.
±or the remainder of the pregnancy,
placental estrogens
and
placental progesterone
maintain the uterine wall. The
placenta also secretes a hormone called
placental lacto-
gen
that may stimulate breast development and prepare the
Trophoblast cells
secrete hCG
Corpus luteum continues
to secrete estrogens and
progesterone
Estrogens and progesterone
promote growth, development,
and maintenance of uterine wall
hCG maintains corpus luteum
024
Months of pregnancy
13579
68
Increasing hormone concentration
Human chorionic
gonadotropin
Estrogens
Progesterone
FIGURE 23.8
Mechanism that preserves the uterine lining during
early pregnancy.
FIGURE 23.9
Relative concentrations of three hormones in
maternal blood during pregnancy.
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