The hormones with the strongest influences on pregnancy are estrogens, progesterone, human chorionic gonadotropin, and human chorionic somatomammotropin. These are
37 neo = new + nat = born, birth secreted primarily by the placenta, but the corpus luteum is an important source of hormones in the first 7 to 12 weeks. If the corpus luteum is removed before the seventh week, abortion almost always occurs. From weeks 7 to 17, the corpus luteum degenerates and the placenta takes over its endocrine functions.
Human chorionic gonadotropin (HCG) is secreted by the blastocyst. Its presence in the urine is the basis of pregnancy tests and can be detected with home testing kits as early as 8 or 9 days after conception. HCG secretion peaks around 10 to 12 weeks and then falls to a relatively low level for the rest of gestation (fig. 28.18). Like LH, it stimulates growth of the corpus luteum, which doubles in size and secretes increasing amounts of progesterone and estrogen. Without HCG, the corpus luteum would atrophy and the uterus would expel the conceptus.
Estrogen secretion increases to about 30 times normal by the end of gestation. The corpus luteum is an important source of estrogen for the first 12 weeks; after that, it comes mainly from the placenta. The adrenal glands of the mother and fetus secrete androgens, which the placenta converts to estrogens. The most abundant estrogen of pregnancy is estriol, but its effects are relatively weak; estra-diol is less abundant but accounts for most of the estro-genic effects in pregnancy.
Estrogen stimulates tissue growth in the fetus and mother. It causes the mother's uterus and external genitalia to enlarge, the mammary ducts to grow, and the
Saladin: Anatomy & I 28. The Female I Text I I © The McGraw-Hill
Physiology: The Unity of Reproductive System Companies, 2003 Form and Function, Third Edition breasts to increase to nearly twice their former size. It makes the pubic symphysis more elastic and the sacroiliac joints more limber, so the pelvis widens during pregnancy and the pelvic outlet expands during childbirth.
The placenta secretes a great deal of progesterone, and early in the pregnancy, so does the corpus luteum. Progesterone and estrogen suppress pituitary secretion of FSH and LH, thereby preventing more follicles from developing during pregnancy. (This is the basis for contraceptive pills and implants; see insight 28.4, p. 1079.) Progesterone also suppresses uterine contractions so the conceptus is not prematurely expelled. It prevents menstruation and promotes the proliferation of decidual cells of the endometrium, on which the blastocyst feeds. Once estrogen has stimulated growth of the mammary ducts, progesterone stimulates development of the secretory acini— another step toward lactation.
Human Chorionic Somatomammotropin
(HCS) secreted in pregnancy is several times that of all the other hormones combined, yet its function is the least understood. The placenta begins secreting HCS around the fifth week and HCS output increases steadily from then until term, in direct proportion to the size of the placenta.
HCS is sometimes called human placental lactogen because, in other mammals, it causes mammary development and lactation; however, it does not induce lactation
in humans. Its effects seem similar to those of growth hormone, but weaker. It also seems to reduce the mother's insulin sensitivity and glucose usage such that the mother consumes less glucose and leaves more of it for use by the fetus. HCS promotes the release of free fatty acids from the mother's adipose tissue, providing an alternative energy substrate for her cells to use in lieu of glucose.
Pregnancy affects many other aspects of endocrine function. A woman's pituitary gland grows about 50% larger during pregnancy and produces markedly elevated levels of thyrotropin, prolactin, and ACTH. The thyroid gland also becomes about 50% larger under the influence of HCG, pituitary thyrotropin, and human chorionic thyrotropin from the placenta. Elevated thyroid hormone secretion increases the metabolic rate of the mother and fetus. The parathyroid glands enlarge and stimulate osteoclast activity, liberating calcium from the mother's bones for fetal use. ACTH stimulates glucocorticoid secretion, which may serve primarily to mobilize amino acids for fetal protein synthesis. Aldosterone secretion rises and promotes fluid retention, contributing to the mother's increased blood volume. The corpus luteum and placenta secrete relaxin, which relaxes the pubic symphysis in other animals but does not seem to have this effect in humans. In humans, it synergizes with progesterone in stimulating the multiplication of decidual cells in early pregnancy and promotes the growth of blood vessels in the pregnant uterus.
The hormones of pregnancy are summarized in table 28.2.
Table 28.2 The Hormones of Pregnancy
Human chorionic gonadotropin (HCG) Estrogens
Human chorionic somatomammotropin (HCS)
Pituitary thyrotropin Human chorionic thyrotropin Parathyroid hormone Adrenocorticotropic hormone Aldosterone Relaxin
Prevents involution of corpus luteum and stimulates its growth and secretory activity; basis of pregnancy tests
Stimulate maternal and fetal tissue growth, including enlargement of uterus and maternal genitalia; stimulate development of mammary ducts; soften pubic symphysis and sacroiliac joints, facilitating pelvic expansion in pregnancy and childbirth; suppress FSH and LH secretion Suppresses premature uterine contractions; prevents menstruation; stimulates proliferation of decidual cells, which nourish embryo; stimulates development of mammary acini; suppresses FSH and LH secretion
Weak growth-stimulating effects similar to growth hormone; glucose-sparing effect on mother, making glucose more available to fetus; mobilization of fatty acids as maternal fuel Stimulates thyroid activity and metabolic rate Same effect as pituitary thyrotropin Stimulates osteoclasts and mobilizes maternal calcium
Stimulates glucocorticoid secretion; thought to mobilize amino acids for fetal protein synthesis Causes fluid retention, contributing to increased maternal blood volume Promotes development of decidual cells and blood vessels in the pregnant uterus
Saladin: Anatomy & I 28. The Female I Text I I © The McGraw-Hill
Physiology: The Unity of Reproductive System Companies, 2003 Form and Function, Third Edition
1072 Part Five Reproduction and Development
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