A normal menstrual cycle lasts between 24 and 38 days, counted from the first day of one period to the first day of the next. The average is 28 days, but plenty of healthy cycles fall shorter or longer than that. If your cycle consistently lands somewhere in that 24-to-38-day window, it’s considered regular, even if it doesn’t hit 28 on the dot.
How to Count Your Cycle Length
Your cycle length isn’t the number of days you bleed. It’s the number of days from the first day of bleeding in one period to the first day of bleeding in the next. So if your period starts on March 1 and your next period starts on March 29, your cycle is 28 days. The bleeding itself typically lasts anywhere from two to seven days, but that’s separate from cycle length.
Tracking at least three consecutive cycles gives you a much better picture than relying on a single month. You can use a calendar, a notes app, or a period-tracking app. Write down the date bleeding starts each time, then count the days between those start dates.
What Counts as Irregular
A cycle shorter than 21 days or longer than 35 days falls outside the typical range. But a single off cycle doesn’t necessarily signal a problem. Stress, travel, illness, and weight changes can all shift your timing by a few days. The pattern matters more than any single month.
Cycles are considered irregular when the length varies by more than 7 to 9 days from one cycle to the next. For example, if one cycle is 26 days and the next is 35, that 9-day swing is worth paying attention to. Consistently irregular timing, bleeding every week for several days in a row, or going three to four months without a period are all patterns that warrant a medical evaluation.
Normal Ranges Change With Age
Teenagers and adults in their 40s operate on different biological clocks, and their cycle lengths reflect that.
Teens and Young Adults
In the first year after a first period, cycles average about 32 days but can range anywhere from 21 to 45 days. That wide window exists because the hormonal signaling system between the brain and ovaries is still maturing. Ovulation doesn’t happen every cycle yet, which means some months stretch longer than others. By the third year after a first period, 60 to 80 percent of cycles settle into the 21-to-34-day adult range.
The Perimenopausal Shift
In your 40s, cycles often start shifting again. Estrogen levels rise and fall less predictably, and ovulation becomes inconsistent. Periods may come closer together for a while, then space out. If the gap between your cycles starts varying by seven days or more from what’s been normal for you, that’s often an early sign of perimenopause. Going 60 days or more between periods typically signals late perimenopause. These changes can start as early as your mid-30s, though they’re more common in the mid-to-late 40s.
How Birth Control Changes the Pattern
Hormonal birth control overrides your natural cycle, so the “period” you get on the pill isn’t a true period. It’s withdrawal bleeding that happens during the hormone-free days of your pack. On a standard pill pack, this bleeding comes roughly every 28 days because the pack is designed that way.
Extended-cycle pills work differently. Some are designed so you take active hormones for 84 days (12 weeks) and then have one week of inactive or low-dose pills, giving you a period about once every three months. Continuous-use options skip the breaks entirely, meaning you may not bleed at all for a year or longer. If you’re on hormonal birth control, the interval between bleeding episodes is dictated by the medication schedule rather than your body’s own hormonal rhythm.
Hormonal IUDs and implants can also lighten or eliminate periods over time, making cycle tracking less straightforward. If you’re using any of these methods, the typical 24-to-38-day range doesn’t apply in the same way.
What Shorter or Longer Cycles Can Mean
Consistently short cycles (under 21 days) can point to thyroid issues, hormonal imbalances, or changes in ovulation patterns. They can also mean you’re losing more blood overall each month, which raises the risk of iron deficiency over time.
Consistently long cycles (over 35 days) are often linked to conditions like polycystic ovary syndrome (PCOS), where ovulation is delayed or absent. Thyroid dysfunction, high stress levels, significant weight loss, and excessive exercise can also push cycles longer. Going three to six months without a period when you’re not pregnant, breastfeeding, or on hormonal birth control is a separate category that needs investigation.
Some variation from month to month is completely normal. A 26-day cycle one month and a 29-day cycle the next is nothing to worry about. The red flags are consistent extremes, large swings between cycles, or bleeding patterns that don’t follow any recognizable rhythm, like spotting every week or bleeding in unpredictable bursts.