HORMONAL
FLUCTUATIONS IN PREGNANCY
In addition to its function as the agent of transfer of gases and nutrients, the placenta also has significant endocrine activity. It produces progesterone, which is important in maintaining the pregnancy; somatomammotropin (also known as placental lactogen), which acts to increase the amount of glucose and lipids in the maternal blood; estrogen; insulin-like growth factors; relaxin; and –human chorionic gonadotrophin (β-hCG). This hormonal activity is the main cause of the increased maternal blood glucose levels seen in pregnancy, which results in an increased transfer of glucose and lipids to the fetus.
The corpus luteum of the ovary
secretes estrogen and progesterone until the fourth month of gestation in
amounts only slightly higher than those produced after ovulation in the second
half of the regular cycle. However, not later than the 60th day of gestation,
the placenta begins to secrete these hormones in progressive quantities, which
reach their maximum at the end of gestation. Chorionic gonadotropin rescues the
corpus luteum from programmed demise and ensures that progesterone and estrogen
are secreted in early pregnancy, until about 2
months of gestation when the placenta takes on the role of producing sufficient
progesterone and estrogen. Because of the marked production of estrogen and
progesterone by the placenta, bilateral oophorectomy after the fourth month of
pregnancy does not usually alter the course of gestation.
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| Plate 12-7 |
The site of formation of estrogen
and progesterone in the placenta is the syncytial layer of the trophoblast. The
production of progesterone by the trophoblast begins to decrease during the
last month of gestation. This decrease is related to the cause of onset of
labor, though the full nuances of the triggers of labor remain to be
elucidated.
Chorionic gonadotropic hormone is
secreted from the placenta soon after implantation, reaching its peak during
the third month, after which its levels decrease, first sharply during the
fourth and fifth months, then gradually leveling off until the end of gestation.
Chorionic villi, specifically the syncytiotrophoblasts, are the site of
production of this hormone. In addition to its role in promoting the continuing
production of progesterone by the corpus luteum, it is thought that hCG affects
the immune tolerance of the conceptus. Because of its highly negative charge,
hCG may repel the immune cells of the mother, protecting the fetus during the
first trimester. It has also been hypothesized that hCG may be a placental link
for the development of local maternal
immunotolerance. Chemically, it is a glycoprotein of a relatively large
molecular size, composed of 244 amino acids with a molecular mass of 36.7 kDa. It is heterodimeric, with an subunit
identical to that of luteinizing hormone, follicle-stimulating hormone, and
thyroid-stimulating hormone, and a β subunit that is unique to hCG. A chemiluminescent or fluorometric
immunoassay has been developed that can detect β-hCG levels as low as 5 mIU/mL and allows quantitation of the β-hCG concentration.
In early pregnancy, the function of
this hormone seems to be aimed at keeping up the activity of the corpus luteum
through interaction with the transmembrane receptor, resulting in continuing
progesterone secretion, which is needed for the decidualization of the
endometrium. After the fetus and placenta are well developed, the need for a
corpus luteum, and therewith for this gonadotropic hormone, becomes less
imperative.
The excretion rates of
gonadotropins, estrogens, and pregnanediol vary to a great extent. The curves
in the picture represent only an approximate graphic demonstration of the
excretion changes during gestation rather than exact values at given times. For
this reason, no scale has been entered. Not shown are the excretion values of adrenal
cortical hormones, which are also increased
during pregnancy.
