You bleed on your period because the lining of your uterus, built up over the previous weeks to support a potential pregnancy, breaks down and sheds when pregnancy doesn’t occur. The trigger is a drop in the hormone progesterone, which sets off a chain reaction of blood vessel constriction, tissue breakdown, and ultimately bleeding that lasts anywhere from three to seven days. What comes out isn’t purely blood, either. On average, blood makes up only about 36% of menstrual fluid. The rest is mostly tissue fluid from the broken-down lining itself.
The Hormonal Trigger
Each month after ovulation, a temporary structure on the ovary called the corpus luteum pumps out progesterone. This hormone keeps the uterine lining thick, spongy, and rich with blood vessels, essentially maintaining a hospitable environment for a fertilized egg. If no pregnancy happens within about two weeks, the corpus luteum breaks down and progesterone levels plummet.
That progesterone withdrawal is the single event that initiates your period. It’s the hormonal equivalent of pulling the plug. Without progesterone’s stabilizing effect, the lining can no longer sustain itself, and the body begins dismantling it in a surprisingly organized sequence.
How the Lining Actually Breaks Down
The uterine lining has its own dedicated blood supply: tightly coiled vessels called spiral arteries that feed the upper layer of tissue. When progesterone drops, these arteries constrict and coil even tighter, choking off blood flow to the top two-thirds of the lining. This starves the tissue of oxygen, causing it to die.
At the same time, the progesterone drop unleashes enzymes that dissolve the structural framework holding the tissue together. Immune cells flood into the lining and release inflammatory signals, amplifying the breakdown. The lining’s own cells participate in this process, essentially programming their own destruction but limiting it to the upper functional layer. The deeper base layer stays intact so the lining can regenerate after your period ends.
Finally, the damaged tissue releases prostaglandins, compounds that make the uterine muscle contract. These contractions help push the loosened tissue and blood out through the cervix and vagina. Prostaglandins are also why periods come with cramps: stronger contractions mean more pain.
What’s Actually in Menstrual Fluid
Period fluid looks like blood, but it’s a mix. Studies measuring the actual composition found that blood accounts for an average of 36% of what you see, though this varies enormously between individuals, from as little as 2% to over 80%. The bulk of the fluid comes from the endometrial tissue itself as it breaks apart and liquefies. Small amounts of cervical and vaginal secretions mix in as well.
A typical period produces less than 60 mL of blood total across all days, which is roughly four tablespoons. Anything over 80 mL per cycle is considered clinically heavy. Periods lasting longer than seven days or producing frequent large clots are independent signs of heavy menstrual bleeding worth discussing with a provider.
Why the Color Changes Day to Day
Period blood changes color because of oxidation, the same chemical reaction that turns a cut apple brown. The longer blood sits in the uterus before exiting, the darker it gets.
- Pink: Common on day one, when fresh bright red blood mixes with the clear or milky vaginal discharge your body was already producing. Very light flow can also appear pink.
- Bright red: Fresh blood that moved through the uterus and out quickly, typical of the early, heavier days of your period.
- Dark red: Older blood that took longer to exit. You may notice it becoming thicker or containing small clots, which form when blood pools in the uterus long enough to clump together.
- Brown: The most oxidized blood, usually appearing in the last day or two as the uterus clears out what remains. It often mixes with discharge, giving it a dark brown appearance.
Small clots are normal. They simply mean blood sat in the uterus before passing. Changes in clot size or number, though, can signal that something has shifted in your cycle.
Why the Body Sheds Instead of Reabsorbing
Most mammals don’t have periods. When their uterine lining isn’t needed, their bodies quietly reabsorb the tissue. Humans, along with a handful of other primates, bats, and elephant shrews, shed it instead. This has puzzled biologists for decades.
The leading explanation centers not on menstruation itself but on what happens before it. In humans, the uterine lining transforms each cycle in preparation for pregnancy without waiting for any signal from an embryo. This preemptive preparation, called spontaneous decidualization, likely evolved because of a conflict of interest between mother and fetus. An embryo benefits from burrowing deeply into a nutrient-rich lining, but a mother benefits from being able to screen embryos and control how much resources they take. By transforming the lining in advance, the body can evaluate an embryo on its own terms.
Menstrual bleeding is the cost of this system. Once the lining has transformed and no pregnancy occurs, the tissue is too specialized to simply be reabsorbed. Shedding it and rebuilding from scratch each cycle is the body’s solution. It’s metabolically expensive and sometimes inconvenient, but it’s a side effect of a reproductive strategy that gives the body more control over pregnancy.
The Full Cycle in Context
Your period is just the opening act of a roughly 28-day cycle, though anywhere from 21 to 35 days is normal. During the bleeding phase (days one through five or so), the base layer of the lining is already beginning to regenerate. Rising estrogen levels over the next week or two drive the lining to thicken again. Ovulation typically occurs around the midpoint of the cycle, after which progesterone takes over to mature and stabilize the lining. If no embryo implants, progesterone drops, and the whole process resets.
The cycle is coordinated by a feedback loop between the brain and the ovaries. The hypothalamus and pituitary gland in the brain send hormonal signals that control ovulation and hormone production, while the ovaries send signals back that fine-tune those brain hormones. This is why stress, illness, weight changes, and other factors that affect brain signaling can also shift your cycle’s timing or flow.