You have a period because your body prepared for a pregnancy that didn’t happen. Each month, your uterus builds a thick, blood-rich lining designed to nourish a fertilized egg. When no egg implants, hormone levels drop, that lining breaks down, and your body sheds it as menstrual blood. The whole process typically repeats every 21 to 35 days and involves a precise chain of hormonal signals between your brain and your reproductive organs.
What Happens Inside Your Body Each Cycle
Your menstrual cycle has three main phases, and each one is driven by shifting hormone levels. In the first phase (the proliferative phase), estrogen rises and signals the lining of your uterus to grow thicker and develop a rich blood supply. This is your body building a cushion, essentially preparing a place for a fertilized egg to land and grow.
After ovulation, when an egg is released from the ovary, you enter the second phase (the secretory phase). A temporary structure on the ovary called the corpus luteum starts producing progesterone, which stabilizes and further prepares the uterine lining. Progesterone is the hormone keeping everything in place.
If the egg isn’t fertilized, the corpus luteum breaks down after about two weeks and progesterone levels plummet. That drop is the direct trigger for your period. Without progesterone, the spiral-shaped blood vessels in the uterine lining begin to constrict and coil tightly, cutting off blood flow to the upper layers of tissue. Starved of oxygen, those layers start to break apart. The tissue detaches, blood vessels open up, and the mixture of blood, tissue, and mucus exits through the cervix and vagina. That’s your period.
The shedding process is a carefully orchestrated cycle of inflammation, tissue breakdown, clotting, and then repair. Within days of your period starting, the lining is already beginning to regenerate for the next cycle.
How Much Bleeding Is Normal
A typical period lasts about four to five days, and the total blood loss is surprisingly small: around 10 to 35 ml for the entire period, or roughly two tablespoons. It often looks like more because menstrual fluid also contains tissue and mucus, not just blood.
Periods lasting longer than seven days or requiring a new pad or tampon every one to two hours are considered heavy. Other signs of unusually heavy flow include passing blood clots the size of a quarter or larger, needing to double up on pads, or waking up to change pads overnight. Heavy bleeding (more than about 80 ml per cycle, or soaking through 16 regular-sized pads or tampons total) is worth discussing with a healthcare provider, since it can lead to iron deficiency and often has a treatable cause.
Why Humans Have Periods at All
Most mammals don’t menstruate. They reabsorb the uterine lining instead of shedding it. Only a handful of species bleed visibly, including humans, some primates, and certain bats. Scientists have proposed several explanations for why.
One early theory suggested menstruation helped flush out bacteria carried into the uterus by sperm. Another proposed that shedding and rebuilding the lining each month actually uses less energy than maintaining it continuously. The more widely accepted current theory focuses on something called spontaneous decidualization, the process by which the uterine lining transforms on its own each cycle regardless of whether an embryo is present. This transformation gives the body a way to screen embryos and protect itself from a deeply implanting pregnancy before the body is ready. In species without menstruation, this transformation only happens after an embryo arrives, giving the body less control.
When Periods Start
The first period, called menarche, has been arriving earlier in recent generations. Among people born between 1950 and 1969 in the U.S., the average age was 12.5 years. For those born between 2000 and 2005, it dropped to 11.9 years. The rates of very early periods (before age 9) more than doubled across those generations.
Rising rates of childhood obesity appear to explain part of this shift, since body fat influences the hormones that trigger puberty. Other contributing factors likely include dietary patterns, psychological stress, adverse childhood experiences, and exposure to endocrine-disrupting chemicals in the environment. A Harvard study found that the trend toward earlier periods was present across all demographics but most pronounced among Black, Hispanic, Asian, and mixed-race individuals, and among those from lower socioeconomic backgrounds.
Why Periods Sometimes Stop
If your period disappears for three or more cycles after being regular, or for six months after being irregular, that’s considered secondary amenorrhea. The most common cause is pregnancy. Breastfeeding and the transition into menopause are the other two physiological reasons periods stop.
Beyond those, five conditions account for the vast majority of cases. Polycystic ovary syndrome (PCOS) is the most common and involves a combination of irregular cycles, excess androgens, and metabolic changes. Functional hypothalamic amenorrhea happens when stress, low body weight, or excessive exercise disrupts the hormonal signals from your brain to your ovaries. High levels of prolactin (a hormone usually associated with milk production) can also suppress your cycle. Primary ovarian insufficiency means the ovaries stop functioning normally before age 40, often accompanied by hot flashes and other menopause-like symptoms. Finally, scar tissue inside the uterus from procedures or infections can physically prevent the lining from shedding normally.
Periods on Birth Control Are Different
If you’re on hormonal birth control, the bleeding you get during your placebo week isn’t a true period. It’s called withdrawal bleeding, and it happens because the steady supply of synthetic hormones pauses, triggering a mild bleed. The key difference: hormonal birth control prevents your uterine lining from thickening the way it normally would, so there’s much less tissue to shed. That’s why bleeding on birth control tends to be lighter and shorter, and why PMS symptoms are often milder.
Birth control manufacturers originally designed the pill with a placebo week specifically to mimic a natural cycle. There’s no biological need for that bleed. It was included to make the pill feel more “normal” when it was introduced in the 1960s. Many modern birth control regimens now skip or reduce the placebo week entirely, and skipping withdrawal bleeding is generally considered safe.