A normal menstrual cycle lasts between 21 and 35 days, counted from the first day of one period to the first day of the next. The average is about 28 days, but plenty of healthy cycles fall shorter or longer than that. What matters most isn’t hitting an exact number, but whether your cycles stay reasonably consistent from month to month.
How to Count Your Cycle Length
Your cycle starts on the first day of bleeding, not when spotting begins. It ends the day before your next period starts. So if you get your period on March 1 and your next period arrives on March 29, that cycle was 28 days long. The days your period actually lasts (typically up to 7 days) are included in that count, not added on top of it.
Tracking a few cycles in a row gives you a much better picture than any single month. You’re looking for your personal pattern. If your cycles bounce between 26 and 30 days, that’s normal variation. If they swing from 25 to 40 days, that inconsistency is worth paying attention to.
What Counts as Irregular
The American College of Obstetricians and Gynecologists considers cycles shorter than 21 days or longer than 35 days to be outside the normal range. But cycle length alone isn’t the whole picture. A cycle is also considered irregular if the gap between your shortest and longest cycles varies by more than 7 to 9 days. For example, if one cycle is 28 days, the next is 37, and the one after that is 29, that level of unpredictability qualifies as irregular even though each individual cycle falls within the 21-to-35-day window.
Going 90 days or more without a period is considered abnormal unless you’re pregnant, breastfeeding, or going through menopause. Missing three or more periods in a row also falls into this category.
Why Cycles Run Short
Cycles shorter than 21 days mean you’re getting your period roughly every two to three weeks. This often relates to hormonal shifts that speed up ovulation or cause the uterine lining to shed earlier than it should. Thyroid problems, particularly an overactive thyroid, can shorten cycles. Stress and sudden changes in weight or exercise habits can also compress cycle length. In some cases, what looks like a short cycle is actually irregular bleeding between periods rather than a true period, which has different causes including infections or growths in the uterus.
Why Cycles Run Long
Cycles longer than 35 days, sometimes called oligomenorrhea, mean you may only get six to eight periods a year. Polycystic ovary syndrome (PCOS) is one of the most common causes. With PCOS, hormonal imbalances can delay or prevent ovulation, stretching the gap between periods. Thyroid dysfunction, both overactive and underactive, can also push cycles longer. Other causes include significant weight changes, hormonal imbalances, certain medications, and high levels of physical or emotional stress.
Intense exercise deserves special mention here. Training hard and regularly can make periods irregular or stop them altogether. This isn’t limited to elite athletes. Starting a vigorous fitness routine after a long stretch of inactivity can have the same effect. A missing period might seem convenient, but it can signal underlying problems including bone density loss and difficulty getting pregnant later.
How Age Changes Your Cycle
Cycle length isn’t static across your lifetime. In the first couple of years after a teenager’s first period, cycles are frequently irregular and can range widely, sometimes stretching to 45 days or more. This is normal. The hormonal feedback system that regulates ovulation takes time to mature.
Through the 20s and 30s, cycles tend to be at their most predictable. Many people settle into a consistent rhythm during these years, though some natural variation from month to month is still expected.
As you approach menopause, things shift again. During perimenopause, which can start in the mid-40s, estrogen levels rise and fall unpredictably. Periods may come closer together, then farther apart. Flow can swing from light to heavy. If your cycle length starts varying by seven days or more from one month to the next, you may be in early perimenopause. Once you’re going 60 days or more between periods, you’re likely in late perimenopause, heading toward menopause itself.
Signs That Need Medical Attention
Some patterns go beyond normal variation. You should get an evaluation if your cycles consistently fall outside the 21-to-35-day range, if you’re missing three or more periods in a row, if your periods last longer than seven days, or if cycle length swings by more than nine days from one month to the next. Bleeding that’s heavy enough to soak through a pad or tampon every hour for several hours is also a reason to get checked.
These patterns don’t necessarily mean something serious is wrong, but they can point to treatable conditions like thyroid disorders, PCOS, or hormonal imbalances that are worth identifying early, especially if you’re planning to get pregnant at some point.