A woman’s menstrual cycle is typically 28 to 30 days long, though anything from 21 to 45 days can be normal. The cycle is counted from the first day of one period to the first day of the next, and the length shifts naturally across different stages of life.
Average Cycle Length by Age
Cycle length isn’t one fixed number. It changes predictably over the reproductive lifespan. A large study from the Harvard T.H. Chan School of Public Health tracked these patterns across age groups and found notable differences.
People under 20 had the longest average cycles at 30.3 days. Cycles gradually shortened through the 20s and 30s, reaching an average of 28.7 days for those aged 35 to 39. The shortest cycles showed up in the early to mid-40s, averaging 28.2 days. After age 50, cycles lengthened again to an average of 30.8 days as the body moved toward menopause.
Regularity follows a similar arc. Cycles tend to be irregular in the first few years after a girl’s first period, then settle into a more predictable rhythm through the 20s and 30s. After age 45, irregularity creeps back in as ovarian function declines. Eventually, menstruation stops permanently after one to three years of long, highly irregular cycles. The average age of menopause in the U.S. is around 52.
How to Count Your Cycle
Day 1 is the first day of your period, meaning the first day of actual bleeding (not spotting). Your cycle length is the number of days from that Day 1 to the day before your next period starts. So if you start bleeding on March 3 and your next period arrives on March 31, your cycle was 28 days.
Tracking for three to six months gives you a much better picture than any single cycle. Most people find their cycles vary by a few days from month to month, and that’s completely normal. Writing down start dates in a calendar or period-tracking app is the simplest way to spot your personal pattern.
What Drives the Timing
Your cycle length is controlled by a sequence of hormonal shifts that unfold in two main phases. The first phase, called the follicular phase, begins on Day 1 of your period. Rising estrogen levels cause the uterine lining to thicken while follicle-stimulating hormone (FSH) prompts your ovaries to develop an egg. This phase is the most variable part of the cycle and is the main reason cycles differ in length from person to person and month to month.
Ovulation happens when a surge of luteinizing hormone triggers one ovary to release a mature egg. After that, the second phase (the luteal phase) begins. Progesterone rises to prepare the uterine lining for a possible pregnancy. This phase is more consistent, lasting about 12 to 14 days in most people. If pregnancy doesn’t occur, estrogen and progesterone both drop, the thickened lining sheds, and your period begins, restarting the cycle.
When a Cycle Is Considered Irregular
Some variation is expected, but certain patterns signal something worth investigating. The American College of Obstetricians and Gynecologists flags these as potential concerns:
- Cycles shorter than 21 days or longer than 45 days
- A gap of more than 90 days between periods, even just once
- Periods that last longer than 7 days
- Cycles that were previously regular and become unpredictable
A single off cycle doesn’t necessarily indicate a problem. But a persistent shift in your pattern, especially if your cycles were previously like clockwork, is worth paying attention to.
What Can Change Your Cycle Length
Beyond age, several factors can shorten, lengthen, or disrupt your cycle. Stress is one of the most common culprits. Physical or emotional stress can delay ovulation, which pushes back the entire cycle and makes your period arrive later than expected.
Significant weight changes in either direction can affect cycle timing. Very low body fat, common in long-distance runners, dancers, and gymnasts, can cause periods to stop entirely. On the other end, excess body weight can contribute to hormonal imbalances that make cycles irregular.
Polycystic ovary syndrome (PCOS) is one of the most common medical causes of irregular or absent periods. Thyroid disorders, both overactive and underactive, also directly affect the hormones that regulate your cycle. Primary ovarian insufficiency, a condition where the ovaries stop functioning normally before age 40, can cause missed or unpredictable periods as well. Certain medications, cancer treatments like chemotherapy and radiation, and some autoimmune conditions round out the list of known disruptors.
If your cycle has always been a bit longer or shorter than the textbook 28 days but stays consistent for you, that’s your normal. The most useful benchmark isn’t a population average. It’s your own pattern over time.