632
Chapter 17
(
miscarriage
) than does amniocentesis. A third technique for
fetal diagnosis is ultrasound, which provides a “picture” of the
fetus without the use of x-rays. A fourth technique for screen-
ing for fetal abnormalities involves obtaining only
maternal
blood and analyzing it for several normally occurring proteins
whose concentrations change in the presence of these abnor-
malities. For example, particular changes in the concentra-
tions of two hormones produced during pregnancy—human
chorionic gonadotropin and estriol—and alpha-fetoprotein (a
major fetal plasma protein that crosses the placenta into the
maternal blood) can identify many cases of
Down syndrome
,
a genetic form of mental retardation associated with distinct
facial and body features.
Maternal nutrition is crucial for the fetus. Malnutrition
early in pregnancy can cause specifi c abnormalities that are
congenital;
that is, existing at birth. Malnutrition retards fetal
growth and results in infants with higher-than-normal death
rates, reduced growth after birth, and an increased incidence
of learning disabilities and other medical problems. Specifi c
nutrients, not just total calories, are also very important. For
example, there is an increased incidence of neural defects in
the offspring of mothers who are defi cient in the B-vitamin
folate (also called folic acid and folacin).
The developing embryo and fetus are also subject to con-
siderable infl uences by a host of non-nutrient factors such as
noise, radiation, chemicals, and viruses, to which the mother
may be exposed. For example, drugs taken by the mother
can reach the fetus via transport across the placenta and can
impair fetal growth and development. In this regard, it must
be emphasized that aspirin, alcohol, and the chemicals in ciga-
rette smoke are very potent agents, as are illicit drugs such as
cocaine. Any agent that can cause birth defects in the fetus is
known as a
teratogen
.
Because half of the fetal genes—those from the father—
differ from those of the mother, the fetus is in essence a
foreign transplant in the mother. The integrity of the fetal-
maternal blood barrier also protects the fetus from immuno-
logical attack by the mother.
Hormonal and Other Changes
During Pregnancy
Throughout pregnancy, plasma concentrations of estrogen and
progesterone continually increase (
Figure 17–29
). Estrogen
stimulates the growth of the uterine muscle mass, which will
eventually supply the contractile force needed to deliver the
fetus. Progesterone inhibits uterine contractility so that the fetus
is not expelled prematurely. During approximately the fi rst two
months of pregnancy, almost all the estrogen and progester-
one are supplied by the corpus luteum.
Recall that if pregnancy had not occurred, the corpus
luteum would have degenerated within two weeks after its
formation. The persistence of the corpus luteum during preg-
nancy is due to a hormone called
human chorionic gonad-
otropin (hCG),
which the trophoblast cells start to secrete
around the time they start their endometrial invasion. hCG
gains entry to the maternal circulation, and the detection of
this hormone in the mother’s plasma and/or urine is used as
a test for pregnancy. This glycoprotein is very similar to LH,
and it not only prevents the corpus luteum from degenerating
but strongly stimulates its steroid secretion. Thus, the signal
that preserves the corpus luteum comes from the conceptus,
not the mother’s tissues.
The secretion of hCG reaches a peak 60 to 80 days after
the last menstruation (see Figure 17–29). It then decreases just
as rapidly, so that by the end of the third month it has reached
a low level that remains relatively constant for the duration of
the pregnancy. Associated with this decrease in hCG secre-
tion, the placenta begins to secrete large quantities of estro-
gen and progesterone. The very marked increases in plasma
concentrations of estrogen and progesterone during the last
six months of pregnancy are due entirely to their secretion by
the trophoblast cells of the placenta, and the corpus luteum
regresses after three months.
An important aspect of placental steroid secretion is that
the placenta has the enzymes required for the synthesis of pro-
gesterone but not those needed for the formation of androgens,
which are the precursors of estrogen. The placenta is supplied
with androgens via the maternal ovaries and adrenal glands and
by the
fetal
adrenal glands and liver. The placenta converts the
androgens into estrogen by expressing the aromatase enzyme.
Human chorionic gonadotropin
Delivery
Estrogen
Progesterone
Maternal levels
Months after beginning of last menstruation
1234567891
0
0
Figure 17–29
Maternal levels of estrogen, progesterone, and human chorionic
gonadotropin during pregnancy. Curves depicting hormone
concentrations are not drawn to scale.
Figure 17–29
physiological
inquiry
Why do progesterone and estrogen levels continue to increase
during pregnancy even though human chorionic gonadotropin
(hCG) decreases?
Answer can be found at end of chapter.
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