The menopause occurs when the supply of responsive oocytes is exhausted. A woman is born with about 1.5 million oocytes. One third are lost by the menarche and the remainder during reproductive life. Most women menstruate about 400 times; in each cycle, some 20 - 30 primordial follicles start to develop and become atretric. As the number of oocytes lost through ovulation is far less than the number present at the menarche, it must be concluded that most are lost through ageing. This probably explains why the age of the menopause is unrelated to the age of menarche or to parity.
The perimenopause is characterised by an increased proportion of anovulatory cycles. As a result, progesterone production is decreased, secretory endometrial changes are less marked, and menses become irregular.
After the menopause, oestrogen production falls as the major source of oestradiol - the granulosa cells of the developing follicle - is lost. Thus negative feedback on pituitary production of FSH and LH by oestrogen is reduced. Serum levels of FSH and LH rise and values greater than 40 IU/l indicate postmenopausal status.
Oestrone becomes the major circulating oestrogen in the postmenopausal women. It is less potent than oestradiol and is derived from peripheral conversion of androstenedione by fat cells. As the levels of oestrone are higher in obese than in thin women, oestrogen level is a less reliable indicator of menopausal status than serum gonadotrophins.
Last reviewed 02/2020