Menopause is the cessation of ovarian function. A woman is “post menopausal” 12 months after her last menstrual period. Ovarian function declines in the 5 years running up to menopause, and this is known as the “perimenopause” or the “menopause transition”. “Premature menopause”, now known as “premature ovarian insufficiency” (POI), occurs when a woman’s ovaries cease functioning under the age of 40.

Menopause affects 100 % of women, usually between the ages of 45–55.

During reproductive life, in regularly ovulating women, the menstrual cycle occurs repeatedly every 4 weeks. As described in Chap. 1, the cycle commences with a batch of follicles starting to develop, and during the follicular phase, oestrogen is secreted, resulting in endometrial proliferation. One follicle becomes the leading follicle, and in an ovulatory cycle, ovulates and then becomes the corpus luteum (CL), which produces progesterone as well as oestrogen. The CL has an inherent life span of about 2 weeks, when, in the absence of a pregnancy it succumbs, resulting in a drop in oestrogen and progesterone levels, which causes in the endometrium to slough (menstrual period). During the menopause transition, a woman has cycles where she makes follicles, but does not ovulate; the follicle still secretes oestrogen which causes endometrial proliferation, and when the follicle undergoes atresia, then oestrogen secretion ceases, and the endometrium is lost – still resulting in menstruation (although in an anovulatory cycle). Once ovarian function totally ceases, there is no folliculogenesis, no oestrogen secreted, no endometrial proliferation or shedding, and amenorrhoea results.