During the second half of the menstrual cycle—the two weeks that begin with ovulation and end with menstruation—the endometrium, or lining of the uterus, prepares for conception. Activated by the ovarian hormones estrogen and progesterone, the lining becomes swollen with blood and glandular tissue. Estrogen, the first female hormone, essentially primes the body for ovulation and fertilization. Progesterone changes the uterine lining, preparing it for the eventuality of nourishing a fertilized egg by turning the lining soft and spongy and increasing it to about ten times its normal thickness.
The cycle begins this way: immediately after menstruation, the hormone FSH (follicle-stimulating hormone) is released from the pituitary gland and stimulates the ovaries to produce estrogen. On approximately the fourteenth day of the cycle, when the estrogen level is sufficiently high, a second hormone—LH, or luteinizing hormone—is released from the pituitary and triggers ovulation, that is, the release of the egg from the ovary. This egg is one of about four hundred eggs, from a reserve of almost half a million eggs, that will ripen during a woman’s lifetime.
If the egg is not fertilized by sperm, resulting in pregnancy, the endometrium follows another course. First, the female sex hormones drop and a third hormone, prostaglandin, is released. Then the enriched endometrial tissue breaks down. The menstrual cycle starts as the uterus begins its rhythmic contractions. The unused endometrial tissue detaches from the womb and is normally flushed out of the body in the form of menstrual blood.