To understand what happens hormonally during perimenopause and menopause, it helps to understand what happens hormonally during the menstrual cycle. Four hormones are primarily responsible for the menstrual cycle: estrogen, progesterone, follicle stimulating hormone (FSH), and luteinizing hormone (LH).
The development of an egg follicle in the ovary is stimulated by FSH. As the egg matures, it secretes estrogen, which stimulates the lining of the uterus (endometrium) to prepare for a fertilized egg by becoming thicker and rich with blood and nutrients. The increased estrogen then suppresses the secretion of FSH (this is referred to as negative feedback loop), which prevents any more eggs from developing during that cycle. Ovulation occurs when a surge in estrogen produces a surge in LH, which causes the follicle to rupture, releasing the egg into the fallopian tube. The ruptured follicle is now known as the corpus luteum, and it secretes progesterone, which helps prepare the endometrium for a fertilized egg. If the egg is fertilized, estrogen and progesterone continue to be released, keeping the endometrium intact. If the egg is not fertilized, the corpus luteum stops producing progesterone and estrogen and progesterone levels drop. Lower levels of these hormones cause the endometrium to shed and menstruation to begin. The cycle begins again as FSH levels rise because the low estrogen level does not suppress it.
During perimenopause, the eggs become less sensitive to FSH and may not develop. If the egg does not develop properly, less estrogen is secreted and levels may not be high enough to cause the LH surge necessary for ovulation. This is known as an anovulatory cycle (a cycle without ovulation). Because the follicle does not rupture, there is no corpus luteum to secrete progesterone. The low levels of estrogen and progesterone cause irregular and/or heavy periods. Menopause occurs due to the loss of the follicles, which leads to decreased levels of estrogen and progesterone.