When you get your period, the lining of your uterus breaks down and leaves your body through your vagina over the course of about 2 to 7 days. The whole process is triggered by a drop in two hormones, estrogen and progesterone, which signals that no pregnancy has occurred that cycle. What follows involves more than just bleeding. Your body releases inflammatory chemicals that cause cramps, digestive changes, and sometimes full-body symptoms that can feel surprisingly flu-like.
Why Your Period Starts
Each month, your uterus builds up a thick, blood-rich lining in preparation for a fertilized egg. If no egg implants, a small hormone-producing structure in your ovary (called the corpus luteum) breaks down after about 14 days. That causes estrogen and progesterone levels to fall sharply. Without those hormones sustaining it, the top layers of the uterine lining lose their blood supply, begin to break apart, and shed. That shedding is your period.
What Your Body Actually Sheds
Menstrual fluid isn’t pure blood. It’s a mix of blood cells, fragments of uterine lining tissue, and vaginal secretions. This is why it often looks and feels different from blood you’d see from a cut. It can be thicker, clumpier, and vary in color throughout your period.
Most people lose less fluid than they think. Normal blood loss for an entire period is under 60 milliliters, roughly 4 tablespoons. Moderately heavy periods fall between 60 and 100 milliliters, and anything over 80 milliliters is considered clinically excessive. Because menstrual fluid contains more than just blood, the total volume coming out will look like more than those numbers suggest.
What the Color of Your Flow Means
The color of your period blood changes based on how quickly it moves through your body. Blood reacts with oxygen and darkens over time, so the shade you see is mostly a clue about flow speed. Bright red blood is fresh, meaning it passed through quickly. This is common at the start of your period or on your heaviest days. As flow slows down, blood has more time to oxidize, turning dark red, then brown, then sometimes nearly black. These darker colors typically show up toward the end of your period and are completely normal.
Why Cramps Happen
To push out the shedding lining, your uterus contracts, and those contractions are what you feel as cramps. The force behind them comes from chemicals called prostaglandins, which are released by cells in the uterine lining just before and during your period. Higher prostaglandin levels mean stronger contractions and more intense pain. This is why over-the-counter anti-inflammatory painkillers (like ibuprofen) can be so effective for cramps: they block your body’s production of prostaglandins, reducing the contractions at their source. Taking them early, before pain peaks, tends to work better than waiting.
Digestive Changes and “Period Flu”
Prostaglandins don’t just stay in your uterus. They circulate through your body and can act on smooth muscle tissue in your digestive tract, causing it to contract or relax in ways it normally wouldn’t. This is the reason many people experience looser stools, diarrhea, or more frequent bowel movements during their period. It’s not in your head, and it’s extremely common.
Some people also develop what’s been called “period flu,” a collection of symptoms that can include nausea, low-grade fever, body aches, and fatigue in the days just before or at the start of menstruation. These symptoms are driven by the same prostaglandins and by broader hormonal shifts. Because the same anti-inflammatory medications that ease cramps also block prostaglandin production throughout the body, they can help with these systemic symptoms too.
How Long a Normal Period Lasts
A typical period lasts between 2 and 7 days. The heaviest flow usually happens in the first 2 to 3 days, then tapers off. Full menstrual cycles (from the first day of one period to the first day of the next) normally run 21 to 35 days. Cycles outside that range, periods lasting longer than 7 days, or bleeding heavy enough to soak through a pad or tampon every hour for several hours are worth bringing up with a healthcare provider.
Your Next Cycle Starts Before Bleeding Stops
One thing most people don’t realize is that your body doesn’t wait for bleeding to finish before preparing for the next cycle. The follicular phase, the part of your cycle where your body begins developing a new egg, starts on day one of your period. While the old lining is still leaving your body, your pituitary gland (a small structure at the base of your brain) begins releasing a hormone that activates your ovaries to start growing new follicles, the tiny fluid-filled sacs where eggs mature. So even on your heaviest day of bleeding, your body is already looking ahead.
This overlap is part of why the menstrual cycle is just that: a cycle. There’s no real pause between one period ending and the next round of preparation beginning. The drop in hormones that triggers your period is the same signal that kicks off the follicular phase, setting the whole process in motion again.