Reproduction
633
The secretion of GnRH and, therefore, of LH and FSH
is powerfully inhibited by high concentrations of progesterone
in the presence of estrogen. Both of these gonadal steroids are
secreted in high concentrations by the corpus luteum and then by
the placenta throughout pregnancy, so the secretion of the pitu-
itary gonadotropins remains extremely low. As a consequence,
there are no ovarian or menstrual cycles during pregnancy.
The trophoblast cells of the placenta produce not only
hCG and steroids, but also inhibin and many other hor-
mones that can infl uence the mother. One unique hormone
that is secreted in very large amounts has effects similar to
those of both prolactin and growth hormone. This protein
hormone,
human placental lactogen,
mobilizes fats from
adipose tissue and stimulates glucose production in the liver
(growth-hormone-like) in the mother. It also stimulates breast
development (prolactin-like) in preparation for lactation. Some
of the many other physiological changes, hormonal and non-
hormonal, in the mother during pregnancy are summarized in
Table 17–10
.
Approximately 5 to 10 percent of pregnant women retain
too much fl uid (edema) and have protein in the urine and hyper-
tension. These are the symptoms of
preeclampsia;
when convul-
sions also occur, the condition is termed
eclampsia
.
These two
syndromes are collectively called
toxemia of pregnancy
.
This
Table 17–10
Maternal Responses to Pregnancy
Response
Placenta
Secretion of estrogen, progesterone, human chorionic gonadotropin, inhibin, human placental lactogen,
and other hormones
Anterior pituitary
Increased secretion of prolactin
Secretes very little FSH and LH
Adrenal cortex
Increased secretion of aldosterone and cortisol
Posterior pituitary
Increased secretion of vasopressin
Parathyroids
Increased secretion of parathyroid hormone
Kidneys
Increased secretion of renin, erythropoietin, and 1,25-dihydroxyvitamin D
Retention of salt and water.
Cause:
Increased aldosterone, vasopressin, and estrogen
Breasts
Enlarge and develop mature glandular structure
Cause:
Estrogen, progesterone, prolactin, and human placental lactogen
Blood volume
Increased
Cause:
Total erythrocyte volume is increased by erythropoietin, and plasma volume by salt and water retention.
However, plasma volume usually increases more than red cells, thereby leading to small decreases in hematocrit.
Bone turnover
Increased
Cause:
Increased parathyroid hormone and 1,25-dihydroxyvitamin D
Body weight
Increased by average of 12.5 kg, 60 percent of which is water
Circulation
Cardiac output increases, total peripheral resistance decreases (vasodilation in uterus, skin, breasts, GI tract,
and kidneys), and mean arterial pressure stays constant
Respiration
Hyperventilation occurs (arterial
P
CO
2
decreases) due to the effects of increased progesterone
Organic metabolism
Metabolic rate increases
Plasma glucose, gluconeogenesis, and fatty acid mobilization all increase.
Cause:
Hyporesponsiveness to insulin due to insulin antagonism by human placental lactogen and cortisol
Appetite and thirst
Increased (particularly after the fi rst trimester)
Nutritional RDAs*
Increased
*RDA—Recommended daily allowance
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