Menopause: 12-Month Amenorrhea Rule

Menopause is defined as the permanent cessation of menstruation resulting from loss of ovarian follicular activity, confirmed retrospectively after 12 consecutive months of amenorrhea in the absence of other pathological or physiological causes. The underlying mechanism is progressive depletion of ovarian primordial follicles: women are born with approximately 1-2 million oocytes, declining to roughly 300,000-400,000 at menarche, with continued atresia until the remaining follicle pool becomes insufficient to sustain cyclic estradiol production and ovulation. During the menopausal transition (perimenopause), declining inhibin B from the shrinking follicular pool reduces negative feedback on the anterior pituitary, causing FSH levels to rise. Elevated FSH initially recruits follicles more rapidly, shortening the follicular phase and menstrual cycle length. As follicular depletion progresses, cycles become increasingly anovulatory with erratic estradiol fluctuations that produce irregular bleeding patterns, vasomotor symptoms, and mood changes. At menopause, estradiol production drops below 20 pg/mL and FSH rises above 30 mIU/mL as the pituitary attempts to stimulate the depleted ovaries. The 12-month amenorrhea rule serves as the clinical diagnostic criterion for menopause in women over 45 — no laboratory testing is...