A period is the bleeding that happens when the lining of your uterus sheds through your vagina, typically once a month. It’s the body’s way of clearing out tissue it built up to support a potential pregnancy. When no pregnancy occurs, hormone levels drop, and that thick lining breaks down and exits the body as menstrual blood. Most periods last 2 to 7 days and arrive every 21 to 35 days.
Why Bleeding Happens
Each month, your uterus builds a soft, nutrient-rich lining on its inner wall. This lining exists for one purpose: to nourish a fertilized egg if one implants. Two hormones, estrogen and progesterone, are responsible for thickening and maintaining that lining.
If no egg is fertilized, your ovaries stop producing those hormones. That sudden drop, especially in progesterone, is the signal that triggers your period. Without hormonal support, the upper layer of the uterine lining breaks apart, and blood vessels in that layer open up. The mixture of blood, tissue, and fluid passes through your cervix and out through your vagina. That’s your period.
The Full Menstrual Cycle
Your period is just one part of a repeating cycle with three main phases.
Follicular Phase
This phase starts on the first day of your period and is the longest, variable phase. While the old lining sheds, your brain sends a signal to your ovaries to start developing small fluid-filled sacs called follicles, each containing an egg. Anywhere from 3 to 30 follicles begin growing, though usually only one will mature fully. As the follicle grows, it produces estrogen, which tells the uterus to start building a fresh lining.
Ovulation
Around mid-cycle, a surge of hormones causes the most mature follicle to release its egg into the fallopian tube. This is ovulation. The window is brief: the egg can be fertilized for only about 12 hours after release. The ovulatory phase itself lasts roughly 16 to 32 hours.
Luteal Phase
After releasing the egg, the empty follicle transforms into a structure that pumps out progesterone, further thickening and stabilizing the uterine lining. This phase lasts about 14 days. If the egg isn’t fertilized, that structure breaks down, progesterone and estrogen plummet, and the cycle resets with another period.
How Much Blood You Actually Lose
A normal period produces less blood than most people assume. Total blood loss for an average period is under 60 milliliters, roughly four tablespoons. Between 60 and 100 milliliters is considered moderately heavy, and anything over 80 to 100 milliliters falls into the category of heavy menstrual bleeding. The fluid you see also contains tissue and mucus, so the total volume of menstrual fluid is greater than the blood component alone.
Signs that your flow may be unusually heavy include soaking through a pad or tampon every hour for several consecutive hours, needing to change products during the night, passing blood clots the size of a quarter or larger, or having periods that last longer than seven days. Heavy bleeding that interferes with daily life or leaves you feeling exhausted and short of breath is worth discussing with a healthcare provider, since it can sometimes lead to anemia.
Why Periods Hurt
Menstrual cramps are caused by chemical messengers called prostaglandins. Your uterus produces these compounds to trigger contractions that squeeze the lining out. The contractions temporarily compress blood vessels in the uterine wall, briefly cutting off oxygen to the tissue, which is what creates that cramping pain in your lower abdomen.
Some people produce more prostaglandins than others, which is why cramp severity varies so much from person to person. Higher levels of these compounds are also linked to heavier periods. Over-the-counter pain relievers that block prostaglandin production (like ibuprofen) tend to work well precisely because they target this mechanism. Taking them at the first sign of cramping, rather than waiting until pain peaks, is generally more effective.
When Periods Start and Stop
Most people get their first period between ages 10 and 15. In the first year or two, cycles are often irregular as the body’s hormonal rhythm establishes itself. Cycles that are longer, shorter, or unpredictable during this window are typical.
Periods continue until menopause, which for most women occurs between ages 45 and 55. In the years leading up to menopause (a transition called perimenopause), cycles often become irregular again, with periods arriving closer together or further apart, and flow becoming lighter or heavier than usual. Menopause is confirmed after 12 consecutive months without a period.
Managing Your Period
Menstrual products vary widely in how much fluid they hold. A 2023 study that tested products with real blood (rather than saline, which previous research had used) found some surprising differences. Tampons held between 20 and 34 milliliters depending on brand and absorbency rating. Heavy-flow pads could hold up to 52 milliliters, well above their advertised 10 to 20 milliliter capacity. Menstrual discs held the most at 61 milliliters on average, with one brand reaching 80 milliliters. Period underwear absorbed the least, holding just 1 to 3 milliliters, and light pads held 3 to 4 milliliters.
Choosing a product comes down to your flow, comfort, and lifestyle. People with lighter flows may do fine with period underwear or light pads, while those with heavier flows often prefer menstrual discs or cups for their higher capacity and longer wear time. Many people use a combination, pairing period underwear with a tampon on heavier days for added protection.
What’s Normal and What’s Not
Cycles that fall anywhere between 21 and 35 days are considered normal, and it’s common for your cycle length to shift by a few days from month to month. Bleeding that lasts 2 to 7 days is typical. Some variation in flow, with heavier days early on tapering to lighter spotting, is the standard pattern.
Patterns worth paying attention to include periods that suddenly become much heavier or more painful than your usual baseline, cycles shorter than 21 days or longer than 35 days on a consistent basis, bleeding between periods, or periods that stop entirely for three or more months when you’re not pregnant. These changes don’t always signal something serious, but they can point to hormonal imbalances, thyroid issues, or structural changes in the uterus like polyps or fibroids that are straightforward to identify and treat.