Cycles are often between 21 and 35 days long. Cycles are called irregular when their length keeps changing a lot from month to month.
Common reasons
Stress, big changes in weight, intense exercise, PCOS, thyroid conditions and the years before menopause can all affect the cycle. Sometimes there is no clear reason.
Irregular cycles and trying
With irregular cycles it is harder to know when ovulation happens. Guidelines suggest regular sex every two to three days rather than timing, and suggest seeing a doctor sooner rather than waiting a full year.
If your periods stop for three months or more and you are not pregnant, or come less than 21 days apart, it is worth talking to a doctor.
Logging your periods in the diary can give your doctor a clear picture of what you noticed.