Ovarian follicles numbers decline gradually throughout a woman’s reproductive years but this process speeds up on approaching MT, falling to a tenth of the level compared to when menstruating regularly. Ultimately follicular development stops and oestradiol production (in the granulosa and thecal cells surrounding the oocyte) is insufficient to stimulate the endometrium, so amenorrhoea occurs.1,2

Follicle stimulating hormone (FSH) is a key hormone in controlling menstruation. During MT, levels usually rise, particularly in the last two years, before plateauing out. Even so, they can fluctuate markedly on a day-to-day basis. Luteinising hormone (LT) secretion from the anterior pituitary also changes in MT, with longer pulses and/or greater amounts produced, increasing LH levels compared to before MT.2

However, a rise in LH levels will not always indicate ovulation. There is also no consistency with FSH/LH ratios in MT: women with premenopausal symptoms may have elevated FSH and LH, elevated FSH with normal LH, or low FSH with high LH.

Luteal progesterone levels normally decline through MT which may reduce ovulation rates and increase the likelihood of prolonged anovulatory cycles. Meanwhile, oestrogen levels having peaked at the age of 35-40 will gently decline until around the age of 45 when they either remain steady or increase during MT.

While the ratios between gonadotrophin and oestrogen metabolite level can fluctuate during MT, oestradiol levels remain unchanged or increase. This all indicates the hypothalamic-pituitary-ovarian feedback loop is desensitised to both the positive and negative feedback effects of oestrogen.2

The result is a wide and unpredictable variation between each cycle, making MT ‘non-linear’ and difficult to assess how much longer it will continue.