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 often-cited “28-day cycle” is just an average, not a standard you need to hit. Your cycle is considered regular as long as it consistently falls somewhere within that 24-to-38-day window.
How Cycle Length Is Counted
Day 1 of your cycle is the first day of full bleeding, not spotting. You count every day from that point until the day before your next period starts. That total is your cycle length. If your period starts on March 1 and your next period starts on March 29, your cycle is 28 days. The days of actual bleeding (typically 3 to 7) are included in that count, not added on top of it.
Why Your Cycle Isn’t Always the Same Length
Your menstrual cycle has two main phases. The first half, before ovulation, is when your body selects and matures an egg. The second half, after ovulation, is a relatively fixed window where your body either prepares for pregnancy or sheds the uterine lining. That second phase typically lasts around 11 days and stays fairly consistent from month to month.
The first half is where almost all the variation happens. A 2024 study in Human Reproduction tracked healthy women over a full year and found that the pre-ovulation phase averaged about 17.6 days but varied significantly more than the post-ovulation phase. So when your cycle comes a few days early or late, it’s almost always because ovulation shifted, not because anything else changed.
How Cycle Length Changes With Age
Your age is one of the biggest factors shaping how many days fall between your periods. Data from the Apple Women’s Health Study at Harvard found a clear pattern across age groups:
- Under 20: Cycles average about 30.3 days and vary by roughly 5.3 days from one cycle to the next. Longer, less predictable cycles are completely normal in the first several years after periods begin.
- Mid-20s to late 30s: Cycles gradually shorten and become more predictable. People ages 35 to 39 had the shortest average cycles (28.7 days) and the least variation, averaging only 3.8 days of difference between cycles.
- 40s: Cycles shorten slightly further (averaging around 28.2 days) but start to become less predictable again, varying by 4 to 11 days as ovarian function begins to decline.
- Over 50: Cycles stretch back out to an average of 30.8 days and become highly variable, with differences of about 11.2 days between cycles. This is the transition toward menopause, which happens at around age 52 on average in the U.S., typically after 1 to 3 years of long and irregular cycles.
What Makes Cycles Shorter or Longer Than Normal
Cycles that consistently fall below 21 days or stretch beyond 35 days are considered outside the normal range. A number of things can push your cycle in either direction.
Short cycles (fewer than 21 days) can result from hormonal shifts during perimenopause, thyroid conditions that speed up your metabolism, or a shortened pre-ovulation phase where your body releases an egg earlier than usual. Stress, significant weight changes, and certain medications can also compress the cycle.
Long cycles (more than 35 days) often point to delayed or absent ovulation. Polycystic ovary syndrome (PCOS) is one of the most common reasons. Thyroid conditions, extreme exercise, very low body weight, and high levels of stress can all delay ovulation enough to stretch a cycle well past 35 days. Hormonal birth control can also change cycle timing or suppress periods entirely.
Some variation from one cycle to the next is normal. But if the gap between your shortest and longest recent cycles is more than 9 days, that level of irregularity is worth paying attention to. For example, a 28-day cycle followed by a 37-day cycle followed by a 29-day cycle would meet that threshold.
After Pregnancy and Breastfeeding
If you’ve recently had a baby, your cycle won’t return on a predictable schedule. Parents who bottle-feed may get their first period as early as five weeks postpartum, and over two-thirds will have a period within 12 weeks. If you’re exclusively breastfeeding, your period typically won’t return until you stop nursing or significantly reduce how often you breastfeed, especially nighttime feeds. Only about one in five breastfeeding parents get a period within six months of giving birth.
When your cycle does come back, expect it to look different for a while. Your first several periods may be heavier, more or less crampy, and irregular in timing. Having one period doesn’t necessarily mean your cycles have fully returned, because you may or may not have ovulated during that first cycle.
Signs Your Cycle Length Needs Attention
Not every irregular cycle signals a problem, but certain patterns are worth investigating. Cycles consistently shorter than 21 days or longer than 35 days fall outside the normal range. Missing three or more periods in a row is considered abnormal unless you’re pregnant, breastfeeding, or going through menopause. Going 90 days or more without a period under any other circumstances also warrants evaluation.
If you previously had regular cycles and haven’t had a period in more than 3 months, or if your periods have always been irregular and you go 6 months without one, that meets the clinical definition of secondary amenorrhea, a condition that can have hormonal, nutritional, or structural causes. Tracking your cycle with a calendar or app for a few months gives you concrete data to bring to any medical conversation, making it easier to spot whether your pattern is normal for you or genuinely shifting.