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.
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.