What counts as an irregular cycle
Your cycle is the time from the first day of one period to the first day of the next. The NHS describes periods as irregular when the gap between them is less than 21 days or more than 35 days. Other health services draw the line in slightly different places, so treat these numbers as a guide.
Irregular periods are more common in the first years after periods start and in the years before menopause. Pregnancy, hormonal contraception, weight changes, stress, too much exercise, polycystic ovary syndrome (PCOS) and thyroid problems can also change your cycle.
How to work out your average when cycles vary
Write down the length of each cycle as it ends. After three or more cycles, add them up and divide by how many you have. If your last three cycles were 26, 34 and 30 days, your average is 30 days.
Also note your shortest and longest cycle. With irregular periods, the range tells you more than the average. In the example above, your next period could come anywhere from around day 26 to day 34, and sometimes outside that.
The irregular period calculator can average your last three cycle lengths for you and gives you a predicted date with a few days of leeway on either side.
What to log besides start dates
- The day your period ends, so you can see if periods are getting longer or shorter
- How heavy the flow is
- Spotting between periods
- Stress, illness, travel or changes in weight or exercise
- Pain or other symptoms that come with your period
With a free Period Tracker account you can log all of this day by day. Once you have logged 3 to 4 periods, predictions are based on your own cycles, and your dashboard can let you know when your cycles look less regular than usual for your age group. How our predictions work explains what changes for irregular cycles.
When your cycles change a lot, ovulation moves too. A calendar alone can't tell you when you ovulate, but a basal body temperature chart can show signs from your own body.
When irregular periods need a doctor
The NHS suggests speaking to a GP if your periods are irregular or your usual pattern changes. See a doctor if:
- Your period lasts longer than 7 days
- You have missed 3 periods in a row
- Your periods have not started by the time you're 15
Your notes will help. Bring the dates of your last few periods and anything else you logged.
Sources
- Irregular periods, NHS
- Missed or late periods, NHS
- Your menstrual cycle, Office on Women's Health