You bleed during your period because the lining of your uterus sheds when your body recognizes that pregnancy hasn’t occurred. Each month, your uterus builds up a thick, blood-rich lining to prepare for a fertilized egg. When no egg implants, hormone levels drop, and that lining breaks down and exits your body through the vagina. The whole process typically lasts 2 to 7 days.
How Your Uterus Builds the Lining
Your menstrual cycle is driven by two key hormones: estrogen and progesterone. In the first half of your cycle, estrogen tells the uterine lining (called the endometrium) to grow thicker. This lining is packed with blood vessels, immune cells, and nutrient-rich tissue, all designed to nourish a potential pregnancy. By mid-cycle, when you ovulate, progesterone takes over and stabilizes the lining, keeping it in place and making it even more hospitable.
If a fertilized egg doesn’t implant within about 10 to 14 days after ovulation, the small gland that released the egg stops producing progesterone. That sudden drop is the trigger for everything that follows.
What Happens When Hormones Drop
Progesterone withdrawal is the direct trigger for menstruation. When levels fall, it sets off a chain of events inside the uterine lining. Specialized blood vessels in the endometrium, called spiral arteries, begin to coil tightly and constrict. This cuts off blood flow to the upper layers of the lining, starving that tissue of oxygen.
At the same time, the drop in progesterone activates enzymes that break down the structural framework holding the lining together. Think of it like removing the scaffolding from a building: once the support is gone, the tissue collapses and detaches from the uterine wall. The damaged blood vessels then open up, releasing blood into the uterine cavity along with the detached tissue.
Your body also releases chemical signals called prostaglandins, which cause the muscular wall of the uterus to contract. These contractions squeeze the broken-down lining out through the cervix and vagina. Prostaglandins are also why periods can be painful: higher levels mean stronger contractions and more cramping.
What’s Actually in Period Fluid
What comes out during your period isn’t pure blood. Roughly half of menstrual fluid is blood from those broken blood vessels in the lining. The other half is a mix of endometrial tissue, cervical mucus, and vaginal secretions. This is why period fluid often looks different from the blood you’d see from a cut: it can be darker, thicker, or more varied in texture.
Your body also releases natural enzymes that act as anticoagulants, preventing the blood from clotting as it leaves the uterus. This is why menstrual blood usually flows freely rather than forming solid clots. On heavier days, though, when blood is shed faster than those enzymes can work, small clots can form. Occasional clots smaller than a quarter are normal.
How Much Bleeding Is Normal
The average period produces about 30 to 80 milliliters of fluid total, spread across 2 to 7 days. That’s roughly 2 to 5 tablespoons. Most people bleed heaviest in the first day or two, then taper off. Cycles themselves typically repeat every 21 to 35 days, counting from the first day of one period to the first day of the next.
There’s a wide range of normal. Some people consistently have light, short periods while others have heavier, longer ones. What matters more than hitting a specific number is whether your period has changed significantly from your own baseline, or whether it’s interfering with your daily life.
Signs Your Bleeding May Be Too Heavy
Heavy menstrual bleeding is one of the most common reasons people seek gynecological care, and it’s worth paying attention to. The American College of Obstetricians and Gynecologists identifies these as signs your bleeding may be excessive:
- Duration: bleeding that lasts more than 7 days
- Saturation: soaking through one or more tampons or pads every hour for several consecutive hours
- Doubling up: needing to wear more than one pad at a time
- Nighttime disruption: having to change pads or tampons during the night
- Large clots: passing blood clots the size of a quarter or larger
Heavy periods can lead to iron deficiency over time, which causes fatigue, weakness, and shortness of breath. Several underlying conditions can cause heavy bleeding, including hormonal imbalances, uterine fibroids (noncancerous growths in the uterine wall), polyps, and clotting disorders. In many cases, the cause is treatable once identified.
Why Your Period Stops on Its Own
Your body doesn’t just passively wait for the bleeding to stop. The same endometrium that just shed begins repairing itself almost immediately. New tissue starts growing over the raw surface of the uterine wall, and the spiral arteries constrict to limit further blood loss. This repair process is remarkably efficient: the uterus heals without scarring, fully restoring its function so the entire cycle can begin again.
Rising estrogen levels in the days after your period fuel this rebuilding. Within a week or so, the lining is already thickening again, and the whole process of preparing for a potential pregnancy restarts. This cycle of building, shedding, and repairing repeats roughly 400 to 450 times over the course of a person’s reproductive life.