Without the formation of a preembryo, the corpus luteum degenerates about two weeks after ovulation as a result of the decline in luteinizing hormone from the anterior lobe of the pituitary gland. The resulting decline in blood levels of estrogens and progesterone causes the endometrium to break down and to be shed with menstruation. Pregnancy causes hormone changes that maintain the endometrium.

The trophoblast of the blastocyst (see figure 18.3) secretes human chorionic gonadotropin (hCG) (ko-re -onS-ik gon-ah-do-tro’-pin), an LH-like hormone that maintains the corpus luteum. Recall that the trophoblast forms the chorion, which forms the embryonic portion of the placenta and is also a source of hCG. Under stimulation of hCG, the corpus luteum continues to secrete progesterone and estrogens to maintain the endometrium. Pregnancy tests are designed to detect the presence of hCG in a woman’s blood or urine, usually within eight to ten days after fertilization.

The concentration of hCG in the blood rises sharply and peaks after approximately 10 weeks of development. Blood hCG levels then decline and level off between 16 and 20 weeks, leading to the degeneration of the corpus luteum. However, by approximately twelve weeks, the placenta takes over the role of producing estrogens and progesterone, which prevents loss of the endometrium when the corpus luteum degenerates. The ovaries remain inactive during the remainder of the pregnancy because the high level of progesterone in the blood suppresses secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus, which in turn prevents the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior lobe of the pituitary.

The blood concentrations of estrogens and progesterone continue to increase throughout pregnancy (figure 18.9). Note that the level of estrogens increase faster than that of progesterone. As pregnancy continues, placental estrogens and progesterone stimulate development of the mammary glands in preparation for milk secretion.