Pregnancy
Pregnancy occurs when the developing embryo embeds itself in the endometrial lining several days following fertilization. During implantation, an embryonic membrane surrounding the embryo begins to produce human chorionic gonadotropic hormone (HCG), which prevents degeneration of the corpus luteum and instead causes it to secrete even larger quantities of progesterone. Progesterone (pro means for; gestation means pregnancy) also inhibits the motility of the uterus. Together with estrogen, it prepares the breasts for lactation (milk production). The corpus luteum may be maintained for as long as six months, even after the placenta is fully developed.
The placenta originates from both maternal and fetal tissue, and is the region of exchange of molecules between fetal and maternal blood, although the two blood types do not mix. After its formation, the placenta continues production of HCG and begins production of progesterone and estrogen. The latter hormones have two effects: They inhibit the anterior pituitary so that no new follicles mature, and they maintain the endometrium so that the corpus luteum is not needed. Menstruation does not occur during the nine months of pregnancy.
Hormonal regulation in the female results in an ovarian cycle. During the first half of the cycle, FSH causes maturation of the follicle, vvhich secretes estrogen. After ovulation and during the second half of the cycle, LH converts the follicle into the corpus luteum, vvhich produces progesterone. Estrogen and progesterone regulate the uterine cycle. Estrogen causes the endometrium to rebuild. Ovulation usually occurs on the fourteenth day of a 28-c1ay cycle. As progesterone is produced by the corpus luteum, the endometrium thickens and becomes secretory. Then, a lovv level of hormones causes the endometrium to break dovvn, as menstruation occurs. Menstruation does not occur during pregnancy because of placental hormones.
