Women have periods because their bodies prepare for pregnancy every cycle, and when pregnancy doesn’t happen, the prepared uterine lining has to go. The uterus builds up a thick, blood-rich layer of tissue each month specifically to nourish a fertilized egg. If no egg implants, hormone levels drop, and that tissue breaks down and exits through the vagina. That’s a period.
It sounds simple, but the process behind it is surprisingly complex, and the deeper question of why humans bleed at all (when most mammals don’t) has a fascinating evolutionary answer.
What the Uterus Does Each Cycle
The inner lining of the uterus, called the endometrium, has two layers. The bottom layer stays relatively stable. The top layer, called the functional layer, is the one that changes dramatically throughout each cycle. It thickens, fills with blood vessels, and becomes spongy and nutrient-rich. This is the layer a fertilized egg would burrow into to establish a pregnancy, and it’s the layer that sheds during a period.
The whole cycle typically runs 21 to 35 days and unfolds in four phases:
- Menstruation: The uterine lining sheds and flows out. Bleeding lasts about five days on average, though anywhere from two to eight days is normal.
- Follicular phase: Hormones signal the lining to start rebuilding while eggs begin maturing in the ovaries.
- Ovulation: A mature egg releases from one ovary, usually around the middle of the cycle.
- Luteal phase: The lining continues thickening in anticipation of a fertilized egg arriving. If none does, hormone levels fall, and the cycle resets with another period.
The Hormone Signal That Triggers Bleeding
The specific trigger for a period is a drop in progesterone. After ovulation, the structure left behind on the ovary (called the corpus luteum) pumps out progesterone, which keeps the uterine lining stable and intact. If pregnancy doesn’t occur, that structure breaks down, progesterone levels plummet, and the thickened lining loses its hormonal support. Without progesterone holding everything together, the blood vessels in the functional layer constrict, the tissue dies, and it sheds.
This is why progesterone is sometimes called the “pregnancy-sustaining” hormone. Its presence means the lining stays. Its absence means the lining goes. The entire system runs on this binary: progesterone up, lining holds; progesterone down, lining sheds.
Interestingly, this only works if the lining was first built up by estrogen earlier in the cycle. Estrogen primes the tissue during the follicular phase, and progesterone stabilizes it afterward. Both hormones are required for a true menstrual period to occur.
Why Humans Bleed When Most Mammals Don’t
Menstruation is rare in the animal kingdom. Most mammals go through something called an estrous cycle, where the uterine lining is simply reabsorbed by the body if pregnancy doesn’t happen. No bleeding, no shedding. Visible menstruation exists primarily in primates (humans, chimpanzees, and other Old World and New World monkeys), along with a few outliers: certain bat species, elephant shrews, and spiny mice.
The key difference comes down to timing. In most mammals, the uterus only transforms its lining into a thick, pregnancy-ready state after a fertilized egg has already arrived and implanted. Humans do it preemptively. Every single cycle, the human uterus thickens its lining and undergoes a process called decidualization before any embryo shows up. The body essentially treats every cycle as a possible pregnancy.
This preemptive preparation has a significant advantage: it gives the uterus more control over which embryos get to implant. Research suggests that the decidualized cells of the uterine lining can actually detect certain abnormalities in a developing embryo. When they identify a problem, they trigger changes that prevent the placenta from forming, which blocks implantation. The abnormal embryo is then removed with the next period. This “choosy uterus” hypothesis is one of the leading explanations for why menstruation evolved. It may be especially important in humans and other primates, which have an unusually high rate of chromosomal abnormalities in embryos.
Other theories have also been proposed. One suggests menstruation helps clear pathogens that sperm can carry into the uterus. Another frames it as an energy-saving strategy: rather than maintaining a thick, metabolically expensive lining all the time, it’s more efficient to shed it and rebuild only when needed.
How Much Blood Is Actually Lost
Periods feel heavier than they are. The most common amount of menstrual flow across a whole period is about two tablespoons (30 ml), and the normal range is 10 to 35 ml. That’s less than a shot glass. The fluid also isn’t pure blood. It’s a mix of blood, tissue from the uterine lining, and mucus, which is why it often looks different from blood you’d see from a cut.
Flow above 80 ml per period, roughly equivalent to soaking through 9 to 12 regular-sized pads or tampons in a single cycle, is considered very heavy. Heavy periods are linked to iron deficiency and anemia. About 37 to 40 percent of women who report heavy menstrual bleeding also have a history of iron deficiency or anemia. The blood lost each month carries iron with it, and when flow is heavy, the body can struggle to replace those iron stores fast enough.
When Periods Start and Stop
The first period (menarche) has been arriving earlier in recent generations. Women born between 1950 and 1969 got their first period at an average age of 12.5. For those born between 2000 and 2005, that average dropped to 11.9, according to a Harvard study. The shift has been more pronounced among racial minorities and lower-income populations, suggesting environmental and nutritional factors play a role alongside genetics.
Periods continue until menopause, which occurs for most women in their late 40s or early 50s. That means the average woman experiences roughly four decades of menstrual cycles, totaling somewhere around 400 to 450 periods over a lifetime.
Bleeding Without Ovulation
Not every “period” is a true period. Technically, menstruation follows ovulation: the egg releases, progesterone rises, no pregnancy occurs, progesterone falls, the lining sheds. But sometimes the body skips ovulation entirely, a situation called anovulation. When this happens, the lining still builds up under the influence of estrogen, but without ovulation there’s no progesterone to stabilize it. The lining can grow too thick and shed irregularly, causing unpredictable bleeding that looks like a period but doesn’t follow the normal hormonal pattern.
This is common during puberty (when cycles are still establishing themselves), during the years approaching menopause, and in conditions like polycystic ovary syndrome. The bleeding is real, but the underlying cycle is different, which is why irregular periods that don’t follow a predictable pattern are worth paying attention to.