Chapter 28
Pregnancy and Human Development
1083
28
Homeostatic Imbalance
28.1
Because many potentially harmful substances can cross placen-
tal barriers and enter the fetal blood, a pregnant woman should
be aware of what she is taking into her body, particularly dur-
ing the embryonic period when the body’s foundations are laid
down.
Teratogens
(ter
9
ah-to-jenz;
terato
5
monster), factors
that may cause severe congenital abnormalities or even fetal
death, include alcohol, nicotine, many drugs (anticoagulants,
sedatives, antihypertensives, and some antibiotics), and ma-
ternal infections, particularly German measles. For example,
when a woman drinks alcohol, her fetus becomes inebriated
as well. However, the fetal consequences may be much more
lasting and result in the
fetal alcohol syndrome
(
FAS
) typified by
microcephaly (small head), mental retardation, and abnormal
growth. Nicotine hinders oxygen delivery to the fetus, impair-
ing normal growth and development. Te sedative
thalidomide
(thah-lid
9
o-mīd) was used by thousands of pregnant women in
the 1960s to alleviate morning sickness. When taken during the
period of limb bud differentiation (days 26–56), it sometimes
resulted in tragically deformed infants with short flipperlike
legs and arms.
Urinary System
Te kidneys produce more urine during pregnancy because of
the mother’s increased metabolic rate and the additional burden
of disposing of fetal metabolic wastes. As the growing uterus
compresses the bladder, urination becomes more frequent, more
urgent, and sometimes uncontrollable (
stress incontinence
).
Respiratory System
Te nasal mucosa responds to estrogens by becoming edema-
tous and congested. Tus, nasal stuffiness and occasional nose-
bleeds may occur. ±idal volume increases markedly during
pregnancy, while respiratory rate is relatively unchanged and
residual volume declines. Te increase in tidal volume is due
to the mother’s greater need for oxygen during pregnancy and
the fact that progesterone enhances the sensitivity of the medul-
lary respiratory center to CO
2
. Many women exhibit
dyspnea
(disp-ne
9
ah), or difficult breathing, during the later stages of
pregnancy.
Cardiovascular System
Te most dramatic physiological changes occur in the cardio-
vascular system. ±otal body water rises, and blood volume may
increase as much as 40% by the 32nd week to accommodate
the additional needs of the fetus. Te rise in blood volume also
safeguards against blood loss during birth. Mean blood pressure
typically decreases during midpregnancy, but then rises to nor-
mal levels during the third trimester. Cardiac output increases
by 35–40% at various stages of pregnancy. Tis helps propel the
greater circulatory volume around the body. Te uterus presses
on the pelvic blood vessels, which may impair venous return
from the lower limbs, resulting in
varicose veins
and leg edema.
gait in the meantime. Considerable weight gain occurs during
a normal pregnancy. Because some women are over- or under-
weight before pregnancy begins, it is almost impossible to state
the ideal or desirable weight gain. However, summing up the
weight increases resulting from fetal and placental growth, in-
creased size of the maternal reproductive organs and breasts,
and greater blood volume during pregnancy, a weight gain of
approximately 13 kg (about 28 lb) usually occurs.
Good nutrition is necessary all through pregnancy if the de-
veloping fetus is to have all the building materials (especially
proteins, calcium, and iron) needed to form its tissues. Addi-
tionally, multivitamins containing folic acid reduce the risk of
having a baby with neurological problems, including such birth
defects as spina bifida and anencephaly, as well as spontaneous
preterm birth. (Indeed, a recent study on preconceptual folic
acid supplementation for a year or more resulted in a reduction
of spontaneous preterm birth by 50–70%.) However, a preg-
nant woman needs only 300 additional calories daily to sustain
proper fetal growth. Te emphasis should be on eating high-
quality food, not just more food.
Not surprisingly, effects of the fetal environment may not show
up until decades later. Below-normal birth weight, for instance,
places females at risk for type 2 diabetes and increases the general
risk of cardiovascular disease later in life for both men and women.
Metabolic Changes
As the placenta enlarges, it secretes increasing amounts of
hu-
man placental lactogen (hPL)
, also called
human chorionic
somatomammotropin (hCS)
. hPL works cooperatively with
estrogens and progesterone to stimulate maturation of the
breasts for lactation, promotes growth of the fetus, and exerts
a glucose-sparing effect in the mother. Consequently, maternal
cells metabolize more fatty acids and less glucose than usual,
sparing glucose for use by the fetus. Gestational diabetes mel-
litus occurs in about 10% of pregnancies, but over half of those
women go on to develop type 2 diabetes later in life.
Plasma levels of parathyroid hormone and activated vitamin
D rise, so that pregnant women tend to be in positive calcium
balance throughout pregnancy. Tis state ensures that the devel-
oping fetus will have adequate calcium to mineralize its bones.
Physiological Changes
Physiological changes take place in many systems during preg-
nancy. A few of these changes are described next.
Gastrointestinal System
Te nausea and vomiting (commonly called morning sickness)
suffered by many women during the first few months of preg-
nancy are believed to be related to elevated levels of hCG, es-
trogens, and progesterone. (Nausea is also a side effect of many
birth control pills.)
Heartburn
, due to reflux of stomach acid
into the esophagus, is common because the esophagus is dis-
placed and the stomach is crowded by the growing uterus.
Con-
stipation
occurs because motility of the digestive tract declines
during pregnancy.
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