Women get periods because their body prepares for pregnancy each month, and when pregnancy doesn’t happen, the uterus sheds the nutrient-rich lining it built up. This cycle of building and shedding is driven by rising and falling hormone levels, primarily estrogen and progesterone, and it repeats roughly every 28 days from puberty until menopause.
What Triggers a Period
Each month, your body goes through a sequence of hormonal shifts designed to create the ideal environment for a fertilized egg. Estrogen rises first, thickening the lining of the uterus (called the endometrium) with blood vessels and tissue. After ovulation, progesterone takes over, further enriching that lining to support a potential embryo.
If no fertilized egg implants, progesterone and estrogen levels drop sharply. This hormonal withdrawal is the direct trigger for menstruation. Without progesterone to sustain it, the thickened lining breaks down and exits the body through the vagina. That’s your period. The whole process then resets and begins again.
The Four Phases of the Cycle
A typical menstrual cycle lasts about 28 days, though anywhere from 21 to 35 days is normal. It moves through four overlapping phases.
Menstruation (days 1 to 5): The uterine lining sheds. Most people bleed for three to five days, though anything from three to seven days falls within the typical range. The endometrium thins down to just 1 to 4 millimeters during this phase.
Follicular phase (days 1 to 13): This phase overlaps with menstruation and continues after bleeding stops. Estrogen rises, prompting the uterine lining to regrow and thicken to about 12 to 13 millimeters. At the same time, follicle-stimulating hormone (FSH) causes small fluid-filled sacs in the ovaries to develop. One of these will mature into a fully developed egg by around day 10 to 14.
Ovulation (around day 14): A surge of luteinizing hormone causes the ovary to release its mature egg. This is the fertile window, the brief period when pregnancy can occur.
Luteal phase (days 15 to 28): The egg travels through the fallopian tube toward the uterus. Progesterone climbs, pushing the endometrium to its peak thickness of 16 to 18 millimeters. If the egg isn’t fertilized, progesterone drops, and the cycle ends with another period.
Why the Body Sheds the Lining Instead of Reabsorbing It
This is one of the more interesting questions in biology. Most mammals don’t menstruate. They simply reabsorb the uterine lining when pregnancy doesn’t occur. Only a handful of species shed it, including humans, some other primates, and certain bats and shrews. Scientists have proposed a few explanations for why.
One long-standing theory is energy conservation. Maintaining a thick, blood-vessel-rich lining month after month would cost the body more metabolic energy than periodically shedding it and rebuilding from scratch. It’s more efficient to let go and start over.
A second theory involves protection. In menstruating species, the uterine lining transforms on its own each cycle, independent of whether an embryo is present. This transformation may act as a quality-control checkpoint. It allows the body to screen embryos before committing resources to a pregnancy, protecting the mother from investing in a non-viable pregnancy or from the highly invasive way human embryos implant into uterine tissue.
An older hypothesis suggested that menstruation helps flush out infectious agents carried into the uterus by sperm, though this idea has less support today than the energy and embryo-screening theories.
What Menstrual Fluid Actually Contains
Period blood isn’t purely blood. It’s a mix of blood, shed endometrial tissue, mucus, and vaginal secretions. This is why it often looks different from blood you’d see from a cut. It can be darker, thicker, or clumpy, and that’s normal. The composition directly reflects what’s happening inside the uterus as the lining breaks apart.
Most periods involve less blood than people assume. The typical blood volume per cycle is under 45 milliliters, roughly three tablespoons. Blood loss under 60 milliliters is considered normal. Between 60 and 100 milliliters is moderately heavy, and anything over 80 to 100 milliliters qualifies as excessively heavy bleeding. If you’re soaking through a pad or tampon every hour for several hours, or passing large clots regularly, that’s worth investigating.
When Periods Start and Stop
Most girls get their first period between ages 10 and 15, with the average falling around age 12. Early cycles are often irregular as the body’s hormonal patterns establish themselves. It can take a year or two for periods to settle into a predictable rhythm.
On the other end, menopause typically occurs between ages 45 and 55 worldwide, according to the World Health Organization. Periods become irregular and eventually stop as the ovaries produce less estrogen and progesterone. Once you’ve gone 12 consecutive months without a period, menopause is considered complete.
Why Periods Sometimes Stop or Become Irregular
Outside of pregnancy and menopause, several factors can disrupt the cycle. The most common involve changes that affect hormone levels or the signals between the brain and the ovaries.
- Body weight changes: Both obesity and very low body fat (below about 15 to 17 percent) can cause periods to stop. Sudden, significant weight loss from extreme dieting or bariatric surgery is a frequent trigger.
- Excessive exercise: Prolonged, intense physical training without adequate nutrition can suppress the hormones needed for ovulation.
- Stress: Severe anxiety or emotional distress can interfere with the hormonal signals that drive the cycle.
- Polycystic ovary syndrome (PCOS): A common hormonal condition that disrupts ovulation and is one of the leading medical causes of irregular or absent periods.
- Thyroid disorders: An overactive thyroid gland can speed up or suppress the menstrual cycle.
- Certain medications: Some drugs used in cancer treatment or for psychiatric conditions can stop periods as a side effect.
- Premature ovarian insufficiency: In some cases, the ovaries reduce their function before age 40, leading to early loss of periods.
In each of these situations, the underlying issue is usually that the body isn’t producing the right pattern of rising and falling hormones. Without that pattern, the endometrium either doesn’t build up properly or doesn’t receive the progesterone withdrawal signal that triggers shedding. The period is, at its core, a hormonal event. When the hormones are disrupted, the period follows.