Most girls and women get their period every 21 to 35 days, which works out to roughly 11 to 13 periods per year. The often-cited “every 28 days” figure is real but surprisingly uncommon. A large study tracking over 1.5 million women found that only about 16% actually had a median cycle length of 28 days. The rest fell somewhere else within that normal range.
What “Every 28 Days” Really Means
A menstrual cycle is counted from the first day of one period to the first day of the next. The average lands around 29 to 30 days, but healthy cycles can be as short as 21 days or as long as 35. That means some women get their period every three weeks, while others go five weeks between periods, and both are perfectly normal.
Each period itself typically lasts 2 to 7 days of active bleeding. So when people say “how often,” there are really two numbers to think about: the gap between periods (cycle length) and how many days you actually bleed each time.
Why Teen Cycles Are Different
If you’re in your first couple of years of having periods, the rules are looser. Normal teen cycles range from about 20 to 45 days, with an average closer to 32 days. That’s noticeably longer than the adult average, and the variation month to month can be dramatic. Some teens go nearly three months between periods early on, and that’s still within the expected range. The 95th percentile for cycle length in the first year after getting your period is 90 days.
This irregularity happens because the hormonal system driving menstruation is still maturing. Ovulation (releasing an egg) doesn’t happen consistently in the first few years, and without regular ovulation, the timing of periods shifts around. Most cycles settle into a more predictable pattern within two to three years of your first period.
What Triggers Each Period
Your period is the result of a hormonal chain reaction between your brain and ovaries. Each month, estrogen rises and signals the lining of your uterus to thicken in preparation for a potential pregnancy. After ovulation, a second hormone called progesterone takes over to maintain that lining.
If pregnancy doesn’t happen, the structure that released the egg breaks down and stops producing progesterone. That drop in progesterone is the direct trigger for your period. Without it, blood vessels in the uterine lining constrict, the tissue loses its blood supply, and the lining sheds. The whole process then resets, and a new cycle begins.
What Can Change Your Cycle Length
Several everyday factors can make periods come more or less often. Stress is one of the most common culprits. Both physical and psychological stress can suppress the brain signals that regulate your cycle, delaying or even skipping ovulation entirely. When ovulation is delayed, your period comes late. When it’s skipped altogether, your period may not come at all that month.
Intense exercise has a well-documented effect. Women who run more than 50 miles per week, for example, have significantly higher rates of missed periods. But there’s no single exercise threshold that applies to everyone. The effect depends on a combination of training intensity, calorie intake, body weight, body fat, and individual biology. Calorie restriction on its own, even without heavy exercise, can have the same impact.
Significant weight changes in either direction, thyroid problems, and hormonal conditions like polycystic ovary syndrome (PCOS) can all shift cycle length outside the normal range. Hormonal birth control deliberately changes your cycle, and some methods stop periods altogether.
When Irregular Timing Is a Concern
Some variation from month to month is expected, especially in teens and in the years leading up to menopause. But certain patterns consistently fall outside the normal range and are worth getting checked:
- Cycles shorter than 21 days or longer than 35 days (45 days for teens) on a regular basis
- Missing periods for 90 days or more when you’re not pregnant or on hormonal birth control
- Bleeding longer than 7 days per period
- Soaking through a pad or tampon every hour for several hours
- Bleeding between periods that happens consistently
- Severe pain that regularly disrupts your daily life
For teens whose cycles consistently fall outside the 20 to 45 day range, the issue could be related to PCOS, thyroid conditions, eating disorders, or other hormonal imbalances. These are treatable, and catching them early makes a difference.
Tracking Your Own Pattern
The simplest way to know your cycle length is to mark the first day of each period on a calendar or app. After three to four months, you’ll start to see your personal pattern. You’re not looking for a perfect number every month. You’re looking for a general range. If your cycles bounce between 26 and 31 days, that’s a stable, healthy pattern, even though no two months are identical.
Knowing your baseline makes it much easier to spot when something changes. A cycle that’s suddenly 10 days longer than usual after a stressful month is easy to explain. A cycle that’s been gradually getting longer for six months in a row tells a different story and is worth bringing up with a healthcare provider.