Hormonal Control of Menstrual Cycle

The menstrual cycle is regulated by five primary hormones: gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), oestrogen, and progesterone. The hypothalamus synthesises GnRH, which controls the production and release of both FSH and LH from the anterior pituitary gland.

1. Follicular Phase: Following menstruation, GnRH stimulates the release of FSH, which causes an ovarian follicle to grow and secrete oestrogens. This oestrogen repairs the uterine wall by stimulating cell division in the endometrium.
2. Ovulation: When oestrogen concentration reaches a peak near the middle of the cycle, it creates a feedback effect that inhibits FSH and stimulates LH secretion. This rapid increase is known as the LH surge, which triggers ovulation and releases the mature egg into the fallopian tube.
3. Luteal Phase: After ovulation, LH converts the ruptured follicle into a corpus luteum. The corpus luteum secretes both oestrogens and progesterone, which act together to make the endometrium soft, moist, thick, and highly vascularised in preparation for pregnancy. High levels of progesterone inhibit GnRH, thereby stopping further FSH and LH release.
4. Menstruation (Absence of Pregnancy): If fertilisation does not occur, the corpus luteum degenerates, leading to a decline in oestrogen and progesterone. Without this hormonal support, the uterine lining is deprived of nutrients and sloughs off, causing menstruation. The subsequent low level of progesterone allows FSH production to resume, initiating a new cycle.
5. Pregnancy: If the egg is fertilised, the membranes surrounding the implanted embryo secrete chorionic gonadotropin on the sixth day post-ovulation (the 20th day of the cycle). This hormone maintains the corpus luteum, keeping the endometrium thick and suppressing menstruation, while the continued presence of oestrogens and progesterone prevents new eggs from maturing.

• Both LH and FSH reach their peak levels in the middle of the cycle, around the 14th day.
• Hormones such as hCG, hPL, and relaxin are produced in women only during pregnancy.
• In pregnancy tests, the first morning urine contains the highest concentration of hCG.
• The activation of sperm for fertilising the egg is termed capacitation.
• The zona pellucida functions to prevent an increase in the size of the embryo.
• Nervous and emotional influences, such as emotional disturbances, can delay or prevent menstruation.