Women have periods because the body prepares for pregnancy every month, and when pregnancy doesn’t happen, it sheds the preparation. The uterus builds a thick, nutrient-rich lining to support a fertilized egg. If no egg implants, hormone levels drop, and that lining breaks down and exits the body as menstrual bleeding. This cycle repeats roughly every 24 to 38 days from puberty until menopause.
What the Uterus Builds Each Month
The inner lining of the uterus, called the endometrium, has two layers. The deeper layer is a permanent base that stays in place throughout your life. The surface layer is the one that changes. Each month, in response to rising hormone levels, this surface layer thickens with blood vessels, fluid, and nutrients designed to nourish an embryo if a fertilized egg arrives. Think of it as the uterus setting a table for a guest that may or may not show up.
By the time ovulation occurs (when an egg is released from the ovary, usually around the middle of the cycle), this lining is plush and ready. If the egg is fertilized and implants, the lining stays and becomes part of early pregnancy support. If not, the body has no reason to maintain it.
The Hormone Drop That Starts Bleeding
The 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 to maintain the uterine lining. If no pregnancy occurs, that structure breaks down after about 14 days. When it does, progesterone and estrogen levels fall sharply.
Without progesterone holding it in place, the surface layer of the endometrium becomes unstable. Blood vessels in the lining constrict and then break down. The tissue detaches and exits through the cervix and vagina as menstrual blood. The whole process is essentially the body clearing out unused tissue so it can start fresh the next cycle. Physiologically, the menstruating uterus resembles a wound surface that must rapidly repair itself each month.
A normal period involves less than 80 milliliters of blood loss total, roughly the volume of a small cup of espresso spread over several days. In practice, though, no one measures this precisely. Most people gauge their flow as light, normal, or heavy based on how often they change pads or tampons.
Why Periods Come With Cramps
To shed its lining, the uterus has to physically push the tissue out. It does this by contracting, much like it does during labor but on a smaller scale. The contractions are driven by inflammatory compounds called prostaglandins, which the lining releases as it breaks down. The more prostaglandins your body produces, the stronger the contractions and the more intense the cramping.
This is why anti-inflammatory painkillers tend to work well for period cramps. They reduce prostaglandin production, which dials down both the contractions and the pain.
Why Humans Menstruate and Most Mammals Don’t
Menstruation is surprisingly rare in the animal kingdom. Out of roughly 5,500 known mammal species, only a handful menstruate: humans, other higher primates (like chimpanzees and gorillas), certain bat species, elephant shrews, and the common spiny mouse. Every other mammal has a different type of reproductive cycle that doesn’t involve bleeding and often requires an external trigger, like seasonal changes or hormonal signals from a mate, to begin.
The key difference is what happens to the uterine lining when pregnancy doesn’t occur. Most mammals simply reabsorb the lining back into the body. Menstruating species shed it instead. One reason may be that in humans and close relatives, the uterine lining transforms so extensively each cycle, thickening and developing a rich blood supply, that reabsorbing it isn’t practical. Shedding and rebuilding from scratch is the more efficient option.
When Periods Start and Stop
Most girls get their first period between ages 10 and 15, with 12 being a common starting point. In the first year or two, cycles are often irregular because the hormonal system is still maturing. Ovulation doesn’t always happen in these early cycles, which can make periods unpredictable in timing and flow. This is normal.
At the other end, menopause typically occurs between ages 45 and 55. At that point, the ovaries stop releasing eggs and hormone production declines permanently. Periods become irregular and eventually stop altogether. The years leading up to menopause (perimenopause) often bring changes in cycle length, flow, and symptoms as hormone levels fluctuate.
Periods Without Ovulation
You don’t actually need to ovulate to have a period. In cycles where no egg is released, estrogen still stimulates the uterine lining to grow. But without ovulation, there’s no corpus luteum producing progesterone, so the lining never gets the hormonal support it needs to stay stable. Eventually it outgrows its own blood supply or becomes too thick to sustain, and parts of it shed on their own.
These anovulatory cycles are common at the extremes of reproductive life (the teenage years and perimenopause), but they can happen at any age. They tend to produce irregular timing, and when the period does arrive, it’s often heavier than usual because the lining has had extra time to build up. Most women experience at least some irregular cycles like this over their menstrual years.
What Counts as a Normal Period
Current medical guidelines define a normal cycle as one that occurs every 24 to 38 days, measured from the first day of one period to the first day of the next. Bleeding typically lasts between three and seven days. Some variation from month to month is expected, especially during stress, illness, or changes in weight or exercise.
Periods that consistently soak through a pad or tampon every hour for several hours, last longer than seven days, or produce clots larger than a quarter fall outside the normal range. Cycles that arrive more frequently than every 24 days or less often than every 38 days also warrant attention. These patterns don’t always signal something serious, but they’re worth tracking and discussing with a healthcare provider, since they can point to hormonal imbalances, structural issues, or other treatable causes.