At the beginning of menstruation the inhibitory influence of the corpus luteum on the pituitary is removed and FSH is secreted in increasing amounts. This stimulates the growth of the young follicles, and they grow, they release increasing quantities of estrogens. The high estrogen content of the blood causes the pituitary to diminish its production of FSH and increase the output of LH. Ovulation occurs when the balance between FSH and LH has swung sufficiently in favour of LH.
Figure: Diagram showing changes in the endometrium, the ovaries and the circulating ovarian hormones during the menstrual cycle.
There is evidence that small amounts of are produced by the preovulatory follicle, and this hormone may be involved in the ovulatory process perhaps through its action on the brain or the anterior hypophysis. After ovulation, the corpus luteum begins to form in the ruptured follicle under the influence of LH. Gonadotropins activate the corpus luteum and cause it to secrete progesterone and small amounts of estrogen. If a fertilised egg is not produced, functional degeneration of the corpus luteum begins eight to ten days after ovulation. The onset of menstrual bleeding correlates with the withdrawal of progesterone and, to a lesser extent of estrogen in the breakdown of the endometrial blood vessels with subsequent bleeding remain largely unknown. If the egg is fertilised, the pituitary continues to release luteinizing hormone, and the corpus luteum increases in size and augments its output of hormones. Secretory competence of the corpus luteum diminishes slowly after the fourth month of pregnancy, although it remains structurally intact until the end of pregnancy. The placenta, rather than the ovary, is the principal source of progesterone and estrogen during the latter half of pregnancy. Removal of the ovaries after mid pregnancy neither terminates pregnancy nor diminishes the levels of the two types of steroid hormones in the circulation.