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Why do menstrual cycles often vary more before menopause?

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Diagram of the menstrual cycle showing hormone levels, follicle development, and endometrial changes across 28 days. It highlights menstruation, ovulation, and the follicular and luteal phases, with labeled curves for estrogen, progesterone, FSH, and LH. — menstruation Cyclical changes during a woman's normal ovulatory menstrual cycle.

Menstrual cycles often vary in length and flow during the years leading up to menopause due to a transitional phase known as perimenopause. During this time, the ovaries experience a decline in function, causing an increasing number of follicles to fail to rupture and release their ova.12

This failure to ovulate disrupts the normal hormonal regulation of the menstrual cycle. As ovarian activity continues to decline, the production of estrogen and progesterone becomes irregular, which can lead to missed periods, prolonged intervals between cycles, or unpredictable bleeding episodes. These fluctuations are a natural result of the aging process as the body prepares for the permanent cessation of menstruation.21