The commonest cause is the ovarian clock: declining reserve compresses the follicle phase, so cycles shrink from 28 days toward 24 - the classic early perimenopause signal, expected physiology in the 40s.
At younger ages: thyroid overactivity and stress patterns also shorten cycles, and any persistent change deserves a mention. The texture tells: perimenopausal shortening travels with sleep changes, new PMS intensity, and skipped months; thyroid changes bring racing heart, weight loss, heat intolerance. Track three months of dates - the pattern is the information - and bring it in if cycles stay under 24 days or symptoms ride along.
What to do
Common questions
Why is my cycle getting shorter?
The commonest reason cycles shorten steadily is the ovarian clock: as ovarian reserve declines, the follicle phase (the first half of the cycle, the variable part) compresses - eggs get recruited and released sooner - so cycles that ran 28 to 30 days start arriving at 26, then 24. That pattern is the classic early perimenopause signal, and in the 40s it is expected physiology, not a problem in itself. The same shortening at younger ages deserves a look: thyroid overactivity shortens cycles, as can stress-pattern disruptions, and any persistent change in an established pattern is worth mentioning. The distinguishing texture: perimenopausal shortening often travels with companions - sleep changes, new PMS intensity, occasional skipped months, heavier or lighter flow - while thyroid-driven changes bring their own cluster (racing heart, weight loss, heat intolerance). What to do: track three months of dates (the pattern IS the information), and bring the log in if cycles stay under 24 days, the change came suddenly, or other symptoms ride along - the workup is a simple conversation plus thyroid and hormone blood tests. Pymander, a free AI doctor available 24/7 by text, can help you read your pattern before the visit.