A normal menstrual cycle lasts anywhere from 21 to 35 days, counted from the first day of one period to the first day of the next. Bleeding itself typically lasts 2 to 7 days. That’s a wider range than most people expect, and your “normal” can look quite different from someone else’s.
How Cycle Length Changes With Age
Your cycle length isn’t fixed. It shifts predictably over the course of your reproductive years, and knowing what’s typical for your age group can save you unnecessary worry.
Teenagers tend to have longer, more unpredictable cycles. People under 20 average about 30.3 days per cycle, with lengths varying by around 5.3 days from one cycle to the next. This is because the hormonal system driving ovulation is still maturing, so some months it fires on schedule and other months it doesn’t.
Cycles become most consistent in your mid-to-late 30s. People aged 35 to 39 average 28.7-day cycles with only about 3.8 days of variation, making this the most predictable window. In your early 40s, cycles shorten slightly to around 28.2 days on average, but they start becoming less regular again as ovarian function gradually declines. By age 50 and beyond, cycle lengths stretch back out to about 30.8 days on average, with variation jumping to 11.2 days. This increasing unpredictability is a hallmark of the transition toward menopause, which happens around age 52 on average in the U.S. after one to three years of long, highly irregular cycles.
What Happens During Each Phase
Your cycle has three main phases beyond the period itself, each driven by different hormones that prepare your body for a potential pregnancy.
Follicular Phase
This phase starts on the first day of your period and lasts until ovulation. Estrogen levels rise, causing the uterine lining to thicken and build up a blood-rich cushion. At the same time, follicle-stimulating hormone (FSH) triggers small fluid-filled sacs in your ovaries to develop, each one containing an egg. Usually one follicle outpaces the rest and becomes dominant.
Ovulation
A sudden surge of luteinizing hormone (LH) causes the dominant follicle to release its egg. This typically happens around the midpoint of your cycle, though the exact day varies. The egg travels into the fallopian tube, where it can be fertilized for roughly 12 to 24 hours. This is the most fertile window of your cycle.
Luteal Phase
After the egg is released, the empty follicle transforms into a structure that pumps out progesterone. This hormone stabilizes the uterine lining and keeps it in place, ready for a fertilized egg to implant. If pregnancy doesn’t happen, both progesterone and estrogen drop sharply. That hormonal withdrawal causes the thickened lining to break down and shed, which is your period. The luteal phase is the most consistent part of the cycle, usually lasting about 14 days regardless of overall cycle length.
How Much Bleeding Is Normal
Most periods produce less than 45 milliliters of blood, which is about three tablespoons. That can be hard to gauge from a pad or tampon, but as a rough guide, a fully soaked regular tampon holds about 5 milliliters. Blood loss under 60 milliliters is considered normal. Between 60 and 100 milliliters is moderately heavy. Above 80 milliliters is classified as menorrhagia, or excessively heavy periods.
In practical terms, needing to change a pad or tampon every hour or two for several consecutive hours, passing blood clots larger than a quarter, or bleeding through your clothes are signs that your flow may be heavier than typical. Periods that feel “light” or “heavy” compared to your own baseline matter more than hitting an exact number, since what’s normal for you is the most useful comparison.
Signs Your Cycle May Be Irregular
Variation from cycle to cycle is expected, but certain patterns fall outside the normal range. Cycles shorter than 21 days or longer than 35 days, or where the gap between cycles varies by more than nine days, are considered irregular. So is bleeding that lasts longer than seven days, or flow that’s dramatically heavier or lighter than what you’re used to.
Missing three or more periods in a row warrants attention. Going 90 days or more without a period is considered abnormal unless you’re pregnant, breastfeeding, or approaching menopause. Other red flags include bleeding or spotting between periods, spotting after sex, severe pain or cramping accompanied by nausea or vomiting, and foul-smelling vaginal discharge. Cycles that suddenly become very irregular after years of being predictable also deserve a closer look.
What Affects Your Cycle Length
Many factors can shift your cycle without meaning anything is wrong. Stress is one of the most common culprits, because the stress response can delay or suppress the hormonal signals that trigger ovulation. When ovulation is delayed, the whole cycle stretches out.
Significant changes in body weight, intense exercise, travel across time zones, illness, and disrupted sleep can all have similar effects. Hormonal birth control fundamentally alters or overrides your natural cycle, so the “period” you get on the pill is actually a withdrawal bleed rather than a true menstrual period. After stopping hormonal contraception, it can take several months for cycles to regulate.
Conditions like polycystic ovary syndrome (PCOS) and thyroid disorders are among the more common medical causes of persistent irregularity. If your cycles have always been unpredictable and fall well outside the 21-to-35-day window, or if they change suddenly after being regular for years, that pattern is worth investigating rather than assuming it’s just how your body works.