A period happens because your body prepared for pregnancy and it didn’t occur. Each month, the uterus builds a thick, blood-rich lining to support a fertilized egg. When no egg implants, hormone levels drop sharply, and the body sheds that lining through the vagina. That shedding, along with blood and mucus, is your period. The whole process is driven by a precise cycle of rising and falling hormones that repeats roughly every 28 days, though cycles anywhere from 21 to 35 days are normal.
What the Uterus Builds Each Month
The inner lining of the uterus, called the endometrium, has two distinct layers. The deeper layer stays permanently in place and acts as a foundation. The upper layer is the one that changes dramatically throughout each cycle. It thickens, develops new blood vessels, and fills with nutrients, all in preparation for a fertilized egg to attach and grow. If pregnancy doesn’t happen, this entire upper layer breaks down and exits the body. The deeper layer remains intact and immediately begins rebuilding a fresh upper layer for the next cycle.
This building process is fueled by estrogen, which rises during the first half of your cycle. Estrogen triggers the cells of the upper lining to multiply rapidly, roughly tripling the lining’s thickness over about two weeks. After ovulation (the release of an egg from an ovary), a second hormone called progesterone takes over, transforming the lining into a spongy, nutrient-rich environment ideal for embryo implantation.
The Hormone Drop That Triggers Bleeding
If the egg isn’t fertilized, the small structure on the ovary that released it (called the corpus luteum) breaks down about 10 to 14 days after ovulation. When it does, it stops producing progesterone and estrogen. That sudden withdrawal of hormones is the direct trigger for your period. Without progesterone to sustain it, the upper lining loses structural support and begins to break apart.
The drop in hormones also causes specialized blood vessels in the lining, called spiral arteries, to constrict. These tiny arteries coil through the upper layer and supply it with blood. When they tighten, the tissue they feed loses its blood supply and starts to die. The arteries then relax briefly, and blood flows into the damaged tissue, carrying fragments of the lining out through the cervix and vagina. A muscular “sphincter” near the base of these arteries eventually constricts again to slow and stop the bleeding.
Why the Uterus Contracts
Period cramps aren’t random. As the lining breaks down, cells release chemical messengers called prostaglandins, which force the muscular wall of the uterus to contract. These contractions physically squeeze the dying lining away from the uterine wall and push it out. Prostaglandins are necessary for this process, but the amount your body produces varies. Higher levels of prostaglandins cause stronger contractions, which is why some people experience significantly more painful cramps than others. Excess prostaglandins are also linked to heavier bleeding.
How Much Blood Is Normal
A typical period lasts about four to five days and involves losing roughly two to three tablespoons of blood total. That’s less than most people expect. The fluid looks like more because it’s mixed with tissue, mucus, and other secretions from the lining. Heavy menstrual bleeding, where someone loses about twice that amount or bleeds for more than seven days, affects a significant number of people and can lead to iron deficiency over time.
Why Humans Menstruate at All
Most mammals don’t have periods. Instead, they reabsorb the uterine lining if pregnancy doesn’t occur. Only a handful of species menstruate, including humans, some other primates, and certain bats and shrews. Scientists have proposed several explanations for why this difference exists.
One older theory suggested that menstruation helps flush out bacteria carried into the uterus by sperm. Another proposed that shedding and rebuilding the lining uses less energy than maintaining it continuously. The most widely accepted current explanation focuses on something called spontaneous decidualization. In menstruating species, the uterine lining transforms in preparation for pregnancy automatically each cycle, regardless of whether a fertilized egg is present. This transformation is thought to protect the body from overly aggressive embryo implantation, giving the uterus more control over which pregnancies proceed. The tradeoff is that when no embryo arrives, the transformed lining can’t simply be reabsorbed. It has to be shed.
What Happens Without Ovulation
Not every “period” follows the full hormonal sequence described above. Sometimes the ovaries don’t release an egg, a situation called an anovulatory cycle. Without ovulation, the body doesn’t produce the progesterone surge that normally stabilizes and then withdraws from the lining. Instead, estrogen continues building up the lining without the counterbalancing effect of progesterone. Eventually, the lining outgrows its blood supply or becomes unstable and sheds on its own, but irregularly. This bleeding can be lighter or heavier than a true period and often comes at unpredictable times. Technically, it’s not menstruation since no egg was released, but it can look and feel very similar.
Anovulatory cycles are common at the extremes of reproductive life. They happen frequently in the first couple of years after periods begin (typically between ages 10 and 15) and again in the years leading up to menopause, which most people experience between ages 45 and 55. They can also occur during times of high stress, significant weight change, or hormonal conditions like polycystic ovary syndrome.
The Cycle Resets Immediately
Even while you’re still bleeding, the next cycle is already underway. The permanent deeper layer of the endometrium begins generating new tissue within the first day or two of your period. Rising estrogen from a new set of developing follicles in the ovaries stimulates this regrowth. By the time bleeding stops, the upper lining is already measurably thicker. This means there’s no true “resting” phase for the uterus. The entire process, from building to shedding to rebuilding, runs continuously from your first period until menopause, pausing only during pregnancy and sometimes during breastfeeding.