Why do menstrual cycles often become irregular just before menopause?

In the years immediately preceding menopause, women enter a transitional phase known as perimenopause, during which ovarian function begins to decline.1

The primary reason for menstrual irregularity during this time is the failure of ovarian follicles to rupture and release their ova (eggs). Under normal conditions, the menstrual cycle is regulated by the rhythmic maturation of these follicles and the subsequent release of estrogen and [progesterone]. As the supply of healthy follicles diminishes, this hormonal coordination is disrupted, leading to anovulation (failure to ovulate).12

This hormonal instability manifests as irregular bleeding patterns, such as missed periods, prolonged intervals between cycles, or variations in the duration and intensity of menstrual flow. As perimenopause progresses toward the final cessation of menstruation, the continued decline in ovarian activity and the rapid drop in estrogen levels in the final years further contribute to these irregularities.1