You bleed during your period because the inner lining of your uterus sheds when your body recognizes that pregnancy hasn’t occurred. Each month, hormones build up a thick, blood-rich layer of tissue inside the uterus to support a potential embryo. When no embryo implants, hormone levels drop, and that tissue breaks down and exits through the vagina as menstrual fluid. The whole process typically lasts three to seven days.
How Hormones Start the Process
The menstrual cycle runs on two key hormones: estrogen and progesterone. During the first half of the cycle, estrogen drives the uterine lining to thicken and develop new blood vessels. After ovulation, progesterone takes over, stabilizing that lining and making it spongy, nutrient-rich, and ready to receive a fertilized egg.
If no egg implants, the structure that released the egg (called the corpus luteum) breaks down and stops producing progesterone. That sudden withdrawal of progesterone is the trigger for menstruation. Without progesterone holding everything in place, the lining destabilizes, its blood vessels constrict, and the tissue begins to die and detach. In cycles where ovulation doesn’t happen at all, estrogen alone can build up the lining until it becomes unstable and sheds on its own, which is why periods can still occur even without ovulation.
What Happens Inside the Uterus
Your uterine lining has two layers. The deeper layer, closest to the muscle wall, is permanent. It stays in place through every cycle and acts as a regeneration zone. The upper layer is the one your body rebuilds each month and sheds during your period. Only this upper, temporary layer detaches during menstruation. Once the period ends, the permanent base layer immediately begins regrowing a new surface, and the cycle starts again.
The shedding doesn’t happen all at once. It occurs in patches across the uterine surface over several days, which is why bleeding varies in heaviness from day to day rather than arriving as a single event.
Why Spiral Arteries Are Central to Bleeding
The upper layer of the uterine lining is fed by specialized blood vessels called spiral arteries. These coiled arteries are unique to the uterus and play a dual role: they deliver blood to nourish the lining during the cycle, but they also need to shut down quickly when the lining sheds, to prevent excessive blood loss.
As progesterone drops, the body releases compounds that cause these spiral arteries to constrict tightly. This constriction cuts off blood flow to the upper lining, starving the tissue of oxygen. The tissue dies and begins to separate. Then the arteries briefly relax, and blood flows from their exposed, broken ends into the uterine cavity. This cycle of constriction and relaxation is what produces the waves of bleeding you experience during a period.
Cramps and Contractions
Your uterus is a muscular organ, and it actively contracts during menstruation to push the shedding tissue out. These contractions are driven by chemical messengers that increase sharply right before and during your period. The same compounds that constrict the spiral arteries also stimulate the uterine muscle to squeeze.
This is also why periods cause cramps. Higher levels of these chemical messengers mean stronger contractions, which means more intense pain. People with particularly painful periods tend to produce greater amounts of these compounds. The contractions serve a practical purpose: they help expel the detached lining efficiently and push blood through the cervix and out of the vagina.
What Menstrual Fluid Actually Contains
What comes out during your period isn’t pure blood. Menstrual fluid is a mixture of blood, fragments of uterine lining tissue, immune cells, and vaginal secretions. The blood component is typically less than 60 milliliters (about four tablespoons) over the entire period for most people, though it can feel like much more because it’s mixed with these other fluids.
The color of your menstrual fluid tells you something about how quickly it’s moving through your body. Pink discharge at the very start is fresh blood mixing with normal vaginal fluid. Bright red means blood is flowing quickly and hasn’t had time to react with oxygen. As the days pass, blood that pools in the uterus before exiting oxidizes and turns dark red or brown. Brown or very dark blood near the end of your period is simply older blood that took longer to make its way out. All of these colors are normal parts of the same process.
Why Humans Menstruate at All
Most mammals don’t menstruate. Among placental mammals, visible menstruation is limited to humans, apes, Old World monkeys, and a handful of other species like certain bats and elephant shrews. In most mammals, the uterine lining is simply reabsorbed when pregnancy doesn’t occur, with no external bleeding.
The leading scientific explanation centers on something called spontaneous decidualization. In most mammals, the uterine lining only transforms into its thickened, nutrient-rich state in response to an actual embryo implanting. In humans and other menstruating species, the lining transforms on its own every cycle, triggered by progesterone alone, regardless of whether an embryo is present. Researchers at institutions studying reproductive evolution argue this evolved because of a kind of biological conflict between mother and embryo. By pre-transforming the lining, the mother’s body gains the ability to evaluate an embryo before committing resources to it, rather than letting the embryo drive the process. Menstrual bleeding is essentially a side effect of this pre-transformation: when no embryo arrives, the already-transformed tissue has to go somewhere, and shedding it is the result.
Earlier theories suggested menstruation evolved to flush out pathogens carried by sperm or that shedding the lining was cheaper, energy-wise, than maintaining it. Both ideas have been largely set aside because they don’t hold up across species comparisons.
Signs Your Bleeding May Be Too Heavy
Normal menstrual blood loss falls under about 60 milliliters per cycle. Since that’s nearly impossible to measure at home, practical signs of unusually heavy bleeding are more useful. Soaking through a pad or tampon every hour for several consecutive hours, needing to double up on protection, waking at night to change products, bleeding for more than seven days, or passing clots larger than a quarter are all indicators that your flow may be heavier than typical. Persistent heavy periods can lead to fatigue and shortness of breath from gradual blood loss, which is worth flagging to a healthcare provider.