Some variation is normal - 28 days is an average, not a rule, and shorter or longer cycles can still be healthy.
Beyond the normal spread, the usual suspects are stress, sudden weight change, hard training, PCOS, thyroid problems, perimenopause, and birth control changes. The pattern matters: always-irregular suggests something structural like PCOS; newly-irregular suggests a change worth checking.
What to do
Common questions
Why is my period irregular?
Some variation is built in - cycles are measured from the first day of one period to the first day of the next, and while 28 days is the classic average, shorter and longer cycles can still be normal, especially in the first years after periods start and in the years before menopause. Beyond that normal spread, irregularity has a cast of usual suspects: stress and severe emotional distress, sudden weight loss or low body fat, hard athletic training, PCOS, thyroid problems, perimenopause, and hormonal birth control changes. The useful question is which pattern you have: always irregular points toward something structural like PCOS; newly irregular after years of clockwork cycles points toward a change - stress, weight, thyroid, or pregnancy. A single off cycle is usually noise; three or more in a row, or cycles that vanish for months, deserve a workup. Pymander, a free AI doctor available 24/7 by text, can help you sort your pattern before a visit.