During menstruation, the inner lining of the uterus breaks down and exits the body through the vagina, typically over 2 to 7 days. This process is triggered by a sharp drop in hormones after the body confirms no pregnancy has occurred. What feels like a simple bleed is actually a coordinated sequence of inflammation, tissue breakdown, blood vessel constriction, and repair happening simultaneously inside the uterus.
The Hormonal Trigger
Every menstrual cycle, the uterus builds up a thick, blood-rich lining in preparation for a fertilized egg. When no pregnancy occurs, a small hormone-producing structure on the ovary called the corpus luteum breaks down. This causes a rapid decline in progesterone, the hormone that had been maintaining and stabilizing that lining. That progesterone withdrawal is the key event that sets menstruation in motion.
The first day of heavy menstrual bleeding is clinically considered day 1 of your cycle. This marks the beginning of a new cycle, not the end of the old one, which is why period tracking apps ask you to log the first day of full flow rather than spotting.
How the Lining Breaks Down
The shedding process happens in two distinct phases. First, once progesterone drops, specialized cells in the lining respond by releasing inflammatory signaling molecules called prostaglandins and cytokines. These molecules act like an alarm system, calling immune cells into the tissue and triggering swelling. Second, enzymes that can dissolve the structural scaffolding of tissue activate and begin breaking apart the upper two-thirds of the lining, known as the functional layer. The deeper base layer stays intact, which is what allows the lining to regrow during the next cycle.
At the same time, the tiny spiral-shaped blood vessels that feed the lining constrict intensely. This cuts off blood supply to the tissue in short, repeated bursts, similar to what happens when blood flow is temporarily blocked and then restored. The combination of dissolved structural tissue, constricted blood vessels, and immune-driven inflammation causes the lining to detach in pieces and bleed.
What Menstrual Fluid Actually Contains
Menstrual fluid is not purely blood. It’s a mix of blood, vaginal secretions, mucus, and fragments of endometrial tissue from the uterine wall. This is why menstrual fluid often looks and feels different from blood you’d see from a cut. It can be darker, thicker, and vary in texture throughout your period.
The total amount of fluid lost during an average period is about 60 milliliters, or roughly 2.7 ounces. That’s less than a quarter cup. Losing more than 80 milliliters per period is considered heavy menstrual bleeding. Small clots are normal, especially on heavier days, but clots the size of a quarter or larger can be a sign of unusually heavy flow worth mentioning to a doctor.
Why Cramps Happen
The same prostaglandins that help break down the lining also cause the muscular wall of the uterus to contract. These contractions serve a purpose: they help squeeze out the detaching tissue and blood. But they also compress blood vessels in the uterine wall, temporarily reducing oxygen supply to the muscle. That combination of strong contractions and reduced oxygen is what produces the cramping pain most people recognize as period pain.
People who produce higher levels of prostaglandins tend to have more intense cramps. This is why anti-inflammatory pain relievers, which block prostaglandin production, are often effective for menstrual cramps when taken early.
Changes Beyond the Uterus
The cervix, which sits at the lower end of the uterus, shifts during menstruation. It drops lower in the body and opens slightly to allow menstrual fluid to pass through. This is why some people notice the cervix feels easier to reach and softer during their period compared to other times in the cycle.
The inflammatory signals involved in shedding the lining aren’t perfectly contained to the uterus. Prostaglandins can enter the bloodstream and affect other parts of the body, which is why some people experience diarrhea, nausea, fatigue, or general achiness during the first day or two of their period. These symptoms tend to peak when prostaglandin levels are highest, usually on the heaviest flow days.
How Bleeding Stops
Menstruation doesn’t just trail off randomly. The body uses the same clotting system it relies on for any wound. When the small blood vessels in the lining are damaged during shedding, platelets rush to the site and form plugs. A protein mesh called fibrin then reinforces those plugs, sealing off the bleeding points. Simultaneously, the base layer of the endometrium begins regenerating new tissue, driven by rising estrogen levels as the next cycle’s egg follicle starts developing on the ovary.
Most periods last between 2 and 7 days. Bleeding that consistently lasts longer than 7 days falls outside the typical range. Flow usually starts light, becomes heaviest on days 2 or 3, then gradually tapers as clotting and tissue repair take over.
Normal Variation in Flow Patterns
Cycles typically repeat every 21 to 35 days, and what counts as a “normal” period varies significantly from person to person. Color can range from bright red on heavy days to dark brown at the beginning or end, which simply reflects how quickly the fluid is leaving the body. Slower flow has more time to oxidize, turning it darker.
Some cycles are heavier than others even in the same person, influenced by stress, sleep, weight changes, or whether ovulation occurred that month. Cycles where ovulation doesn’t happen tend to build a thicker, more irregular lining, which can result in heavier or more unpredictable bleeding when it finally sheds.