You get your period because an egg released by your ovary wasn’t fertilized, so your body sheds the nutrient-rich lining it built inside your uterus to support a potential pregnancy. This shedding is triggered by a sharp drop in hormones, particularly progesterone. The whole process repeats roughly every 21 to 35 days for most of your reproductive years, from your first period (around age 12 on average) until menopause (around age 49 or 50).
Your Body Prepares for Pregnancy Every Cycle
Your menstrual cycle is essentially your body running through a monthly preparation-and-reset routine. In the first half of the cycle, the hormone estrogen drives the lining of your uterus (the endometrium) to thicken and grow new blood vessels. This lining is made up of surface tissue, connective tissue, specialized blood vessels called spiral arteries, and immune cells. It’s building a cushioned, blood-rich environment where a fertilized egg could implant and grow.
Around the middle of the cycle, a surge of hormones from the pituitary gland in your brain triggers ovulation, releasing an egg from one of your ovaries. After ovulation, the empty follicle that released the egg transforms into a small structure called the corpus luteum, which pumps out progesterone. This hormone shifts the uterine lining from “growing” mode into “ready to receive” mode, making it spongy and nutrient-dense. If a fertilized egg implants, the corpus luteum keeps producing progesterone to sustain the early pregnancy. If no egg implants, the corpus luteum breaks down, and your hormone levels plummet. That’s when your period starts.
The Hormone Drop That Triggers Bleeding
Progesterone withdrawal is the direct trigger for menstruation. When the corpus luteum degrades, progesterone and estrogen levels fall sharply. This hormonal shift sets off a cascade of changes in the uterine lining: the spiral arteries constrict and coil tighter, cutting off blood flow to the upper layers of the endometrium. Without blood supply, those tissue layers become oxygen-starved and begin to break down.
At the same time, your body launches what is essentially a controlled inflammatory response. Immune cells flood into the endometrium, and the tissue releases compounds called prostaglandins. These prostaglandins do two things: they help break down the dying tissue, and they cause the muscular wall of the uterus to contract, pushing the shed lining out. Those contractions are what you feel as period cramps. The more prostaglandins your body produces, the stronger the cramps tend to be.
Only the upper two-thirds of the endometrium sheds. The bottom third, called the basal layer, stays intact and serves as the foundation for rebuilding the lining in the next cycle. This is why the process can repeat month after month for decades without damaging the uterus.
Your Brain Runs the Whole Timeline
The timing of your cycle isn’t controlled by your ovaries alone. It starts in the hypothalamus, a small region at the base of your brain that acts as the cycle’s master clock. The hypothalamus releases a signaling hormone that tells the pituitary gland (also in the brain) to send out two key hormones: one that stimulates egg follicles to grow, and another that triggers ovulation at the right moment.
These brain-ovary signals run on a feedback loop. When estrogen and progesterone levels are high, they signal back to the brain to stop sending growth hormones to the ovaries. When those hormone levels crash at the end of a cycle, the brain registers the drop and starts the process over, sending signals to grow a new batch of follicles. This is how your body resets itself each month. Stress, significant weight changes, and intense exercise can disrupt this signaling chain, which is why those factors can delay or skip a period.
How Long a Normal Period Lasts
A typical period lasts three to seven days. Total blood loss during a period is usually under 60 milliliters, which is about four tablespoons. That might seem like more when you’re experiencing it, because menstrual fluid is a mix of blood, tissue, and mucus, so the total volume of fluid is higher than the actual blood component. Blood loss between 60 and 100 milliliters is considered moderately heavy, and anything over 100 milliliters is classified as excessive.
As for cycle length, data from over 1.5 million women shows that about 91% have cycles between 21 and 35 days. The “textbook” 28-day cycle is actually only the norm for about 16% of women. Cycles of 27 or 29 days are nearly as common. Some variation from month to month is completely normal, especially in the years right after your first period and in the years leading up to menopause.
Why Not Every Bleed Is a True Period
A true menstrual period only happens after ovulation. Sometimes, the body goes through a cycle without releasing an egg. These anovulatory cycles can still produce bleeding, but it works differently. Without ovulation, there’s no corpus luteum, no progesterone surge, and no organized transformation of the uterine lining. Instead, estrogen builds the lining unevenly, and it eventually breaks down in an irregular way.
The difference is noticeable. Bleeding from anovulatory cycles tends to be irregular in timing, unpredictable in flow, and sometimes heavier or more prolonged than a typical period. The usual premenstrual signals, like breast tenderness, bloating, and cramping before bleeding starts, are generally absent because those symptoms are driven by progesterone. Anovulatory cycles are common during puberty and perimenopause, when the hormonal communication between the brain and ovaries is still calibrating or winding down.
The Evolutionary Reason Behind It
From a biological perspective, menstruation isn’t just the body discarding unused tissue. The monthly transformation of the uterine lining appears to serve a protective function. In species that menstruate (which is actually a small group: humans, some other primates, a few bat species, and the elephant shrew), the lining transforms on its own each cycle regardless of whether an embryo is present. This spontaneous transformation is thought to give the body a way to evaluate and control how deeply an embryo can implant, protecting the mother from a pregnancy that could be too invasive or nonviable. Shedding and rebuilding the lining each month essentially gives the uterus a fresh, quality-controlled surface for the next potential pregnancy.
When Periods Start and Stop
Most girls get their first period between ages 11 and 15, with the current U.S. average sitting around age 12. That age has been trending slightly younger over the past few decades, dropping from a median of 12.1 in 1995 to 11.9 between 2013 and 2017. Factors like nutrition, body weight, and genetics all influence when puberty begins.
Periods continue until menopause, which occurs at an average age of about 49 to 50. At menopause, the ovaries stop releasing eggs and hormone production declines permanently, so the monthly cycle of building and shedding the uterine lining ends. Menopause before age 40 is considered premature, and between 40 and 44 is classified as early. In the years leading up to menopause (perimenopause), cycles often become irregular as hormone levels fluctuate more unpredictably, making periods lighter, heavier, closer together, or further apart before they stop altogether.