Your cycle gets shorter as you age, not longer

Through the fertile decades, the average cycle quietly shortens with age. The youngest women have the longest cycles, and they grow steadily shorter through the twenties, thirties, and into the forties. From the youngest group to the oldest, the average loses more than two days. It is the kind of trend you would never spot in your own diary but that is unmistakable across thousands of women.

The variability tells an equally interesting story. In the youngest women, cycles vary the most from month to month. Through the late twenties and thirties that spread tightens and cycles become more predictable. The teenage and early-twenties cycle is the erratic one. The thirties cycle is the metronome.

Average menstrual cycle length by age group, with the typical month-to-month range shaded around it.

Then comes the twist that sets up everything that follows. In the mid-forties the average keeps falling, but the month-to-month variability ticks back up. That small widening is the early signature of the perimenopausal transition starting to scatter a rhythm that had spent two decades settling down. It is the first visible crack in the metronome.

For anyone who has been told to expect long, unpredictable cycles as they age, the real pattern is worth knowing. For most of adult life the cycle gets shorter and steadier, not longer and wilder. The wildness, when it comes, arrives later and means something specific.