A normal menstrual cycle repeats every 21 to 35 days, with bleeding that lasts 2 to 7 days. Most people lose about 2 to 3 tablespoons of blood per period, though it can feel like more. Within that range, there’s a lot of individual variation, and your “normal” may look quite different from someone else’s.
The Four Phases of a Cycle
Your menstrual cycle has two major halves, each driven by different hormones. The first half, called the follicular phase, starts on the first day of your period. During this phase, estrogen rises steadily, thickening the lining of your uterus to prepare for a potential pregnancy. At the same time, follicle-stimulating hormone (FSH) prompts your ovaries to develop egg-containing follicles.
Around the middle of your cycle, a surge in hormones triggers ovulation, releasing a mature egg from one ovary. This is the shortest phase, lasting roughly 24 to 36 hours.
The second half is the luteal phase, which is relatively fixed at 12 to 14 days for most people, though anywhere from 11 to 17 days falls within the normal range. During this phase, progesterone rises to maintain the thickened uterine lining. If the egg isn’t fertilized, both progesterone and estrogen drop sharply, and the lining sheds. That shedding is your period.
Because the luteal phase stays fairly consistent, most of the variation in cycle length comes from the follicular phase. A person with a 28-day cycle and a person with a 34-day cycle likely have similar luteal phases but different times to ovulation.
How Cycles Change With Age
Your cycle at 16 won’t look like your cycle at 35, and that’s expected. A large study from the Harvard T.H. Chan School of Public Health tracked how cycles shift across age groups and found consistent patterns.
In the first few years after a first period, cycles tend to be longer (averaging around 30 days) and more irregular, varying by about 5 days from one cycle to the next. This is because the hormonal feedback system is still maturing. Regularity improves through the twenties and into the thirties. People ages 35 to 39 had the most predictable cycles, with an average variation of only 3.8 days.
After 40, cycles gradually shorten, averaging about 28 days. Then after 45, they start becoming irregular again as ovarian function declines heading into menopause. People over 50 had the widest variation of any group, with cycles differing by an average of 11 days. This transition period, called perimenopause, can last several years and commonly involves skipped periods, heavier or lighter flow, and unpredictable timing.
What Affects Your Cycle
Your cycle depends on a communication chain between your brain and your ovaries. Stress, body weight, and energy balance can all disrupt that chain.
Cortisol, the body’s primary stress hormone, directly interferes with the brain signals that kick off each cycle. During prolonged periods of high stress, your brain reduces its release of the hormones that stimulate ovulation. This explains why a demanding stretch at work, a major life event, or chronic anxiety can delay your period or cause you to skip one entirely.
Body weight plays a role through two appetite-related hormones. Fat cells produce a hormone called leptin that helps maintain regular cycles. When body fat drops too low, leptin falls and the hormonal cascade leading to ovulation weakens. A BMI at or below 17.5 is considered a high-risk factor for losing your period. Even a weight loss of 5% to 10% of body weight in a single month can be enough to disrupt things. On the other end, weight gain and increased fat mass raise leptin levels and can help periods resume.
Intense exercise without adequate fuel creates a similar problem. When energy intake doesn’t match energy output, the body deprioritizes reproduction. This is common in endurance athletes and people with restrictive eating patterns. It’s not the exercise itself that causes irregular periods but the energy deficit behind it.
How Much Bleeding Is Normal
Most periods last 4 to 5 days, though anything from 2 to 7 days is considered normal. Total blood loss is typically 2 to 3 tablespoons across the entire period. That amount can be hard to gauge, so practical signs of heavy bleeding are more useful to track.
Bleeding is generally considered heavy if you’re soaking through a pad or tampon every hour for two to three consecutive hours, passing blood clots larger than a quarter, needing to change protection overnight, or bleeding for more than 7 days. People with heavy menstrual bleeding lose roughly twice the normal amount of blood, which over time can lead to iron deficiency and fatigue.
Signs Your Cycle May Be Outside Normal Range
Some variation from month to month is expected, especially during your teens and forties. But certain patterns are worth paying attention to:
- Cycles shorter than 21 days or longer than 35 days consistently, outside of adolescence and perimenopause
- Bleeding or spotting between periods that isn’t related to hormonal contraception
- Cycle length that changes significantly each month, making your period difficult to predict
- Missing periods entirely for three months or more (if your cycles were previously regular) or six months or more (if they were already irregular)
These thresholds aren’t arbitrary. Three months without a previously regular period is the clinical definition of secondary amenorrhea, meaning it warrants investigation to rule out causes like thyroid issues, hormonal imbalances, or changes in weight and stress.
Tracking What’s Normal for You
Population averages are useful as a reference point, but your own pattern matters more. Tracking your cycle for several months gives you a personal baseline. Note the first day of each period, how many days of bleeding you have, and how heavy the flow is. Even rough notes help you spot meaningful changes versus normal fluctuation.
A cycle that’s consistently 33 days is just as normal as one that’s consistently 26 days. What matters more than hitting a specific number is whether your pattern stays relatively stable and whether you’re ovulating regularly, which consistent cycle timing generally indicates.