A typical menstrual cycle lasts 21 to 35 days in adults, counted from the first day of one period to the first day of the next. The actual bleeding portion lasts 2 to 7 days. But “typical” covers a wide range, and your cycle length can shift depending on your age, health, and lifestyle.
How to Count Your Cycle Length
Day 1 is the first day of full bleeding, not spotting. You count forward from there until the day before your next period starts. That total number of days is your cycle length. If your period starts on March 1 and your next period begins on March 29, your cycle was 28 days. Tracking several cycles in a row gives you a much better picture than relying on a single month, since it’s normal for your cycle to vary by a few days each time.
The Two Phases of Your Cycle
Your cycle has two main phases, separated by ovulation. The first half, called the follicular phase, runs from the start of your period until you ovulate. The second half, the luteal phase, covers ovulation through the day before your next period begins.
These two halves don’t split evenly. The luteal phase is relatively consistent, typically lasting 10 to 15 days. The follicular phase is the variable one. It fluctuates from cycle to cycle and changes across your lifetime, which is why your total cycle length can shift even when nothing is “wrong.” If your cycle runs long one month, it’s almost always because the first half stretched out, not the second.
Cycle Length in Teenagers
Adolescents operate on a wider range. In the first year after a first period, the average cycle runs about 32 days, and cycles anywhere from 21 to 45 days are considered normal. That’s because the hormonal system driving ovulation is still maturing, and many early cycles don’t involve ovulation at all, which tends to make them longer and less predictable.
By the third year after a first period, 60 to 80 percent of cycles fall into the 21-to-34-day adult range. Even so, the occasional long cycle is common in adolescence. What’s statistically uncommon, and worth paying attention to, is going 90 days or more without a period, even once.
How Your Cycle Changes With Age
Cycle length isn’t static across adulthood. Women over 35 tend to have cycles that are about a day shorter on average compared to women under 35, largely because the follicular phase compresses as you get older and ovulation happens a bit sooner.
As you approach menopause, the shifts become more noticeable. In early perimenopause, your cycle length may swing by seven days or more from one month to the next. You might have a 25-day cycle followed by a 38-day cycle. In late perimenopause, gaps of 60 days or more between periods are common, and flow can range from barely there to unusually heavy. These changes can start in your early 40s, though timing varies widely.
What Affects Cycle Length
Several everyday factors can nudge your cycle shorter or longer. Regular alcohol consumption is associated with cycles roughly 1.3 days shorter on average. Smoking has a more dramatic effect on the first half of the cycle: current smokers over 35 have a follicular phase about two days shorter than nonsmokers in the same age group. On the other end, women under 35 who exercise four or more hours per week tend to have a follicular phase about two days longer, which stretches the overall cycle.
Ethnicity plays a role too. Research published in the American Journal of Epidemiology found that Asian women had cycles averaging about 1.7 days longer than Caucasian women, even after adjusting for other factors. Stress, significant weight changes, and shift work can also cause temporary shifts, though these are harder to quantify precisely because they vary so much from person to person.
When a Cycle Is Considered Irregular
A cycle that falls outside 21 to 35 days occasionally isn’t necessarily a problem. Bodies aren’t clocks. But consistently short cycles (under 21 days), consistently long cycles (over 35 days in adults or over 45 days in teens), or a single gap of 90 days or more all fall outside the expected range.
Polycystic ovary syndrome, or PCOS, is one of the most common reasons for persistently long or irregular cycles. About 85 to 90 percent of women with PCOS experience infrequent ovulation, which stretches the gap between periods. In one study, 73 percent of women with PCOS had at least 45 days between periods, and 23 percent went longer than 90 days. The longer the interval between periods, the more pronounced the hormonal imbalances tended to be, including higher levels of androgens and greater insulin resistance.
Thyroid disorders can also shift cycle length in either direction. An overactive thyroid tends to make cycles shorter and lighter, while an underactive thyroid can make them longer and heavier. Both are treatable, and cycle regularity often improves once thyroid levels are managed.
What “Regular” Actually Means
Regular doesn’t mean identical every month. It means your cycles are generally predictable within a few days. If your cycle usually falls between 27 and 31 days, that’s regular, even though it’s not the same number each time. The best way to understand your own pattern is to track at least three to six cycles, noting the start date each time. Over those months, you’ll see your personal range emerge, and that range matters more than hitting an exact number like 28.