The Menstrual Cycle

The menstrual cycle is the roughly 28-day cyclical sequence of hormonal, ovarian and uterine changes that recurs from puberty (the first cycle is called menarche) until menopause (when cycles permanently cease, typically in the late forties or early fifties), preparing the female reproductive tract for a possible pregnancy every cycle and, if pregnancy does not occur, ending in the shedding of the built-up uterine lining as the menstrual flow. The cycle is conventionally described in four phases, with parallel events occurring in the ovary and in the uterus (see the accompanying table).

The cycle is taken to begin with the menstrual phase (roughly days 1–5), during which the built-up endometrium of the previous cycle, no longer supported by adequate hormone levels because the previous cycle's corpus luteum has degenerated, breaks down and is shed along with some blood as the menstrual flow, typically lasting three to five days. As the menstrual phase ends, the follicular phase begins (roughly days 5–13): under FSH stimulation, a fresh cohort of ovarian follicles begins growing, one of which becomes dominant and matures into the Graafian follicle, while in the uterus, rising oestrogen secreted by the growing follicle(s) stimulates the endometrium to regenerate and progressively thicken (the proliferative phase of the endometrium). …