Girls bleed because of menstruation, a monthly process where the uterus sheds its inner lining when pregnancy hasn’t occurred. The average person loses less than 60 mL of blood per cycle (roughly a few tablespoons), and a typical period lasts 3 to 7 days. This bleeding is a normal part of the reproductive cycle that begins during puberty, usually around age 11 to 12.
What Actually Happens Inside the Body
The uterus has two layers. The deeper layer stays in place permanently, but the upper layer, called the functional layer, rebuilds and sheds every cycle. Each month, the body prepares this upper layer to receive and nourish a fertilized egg. It thickens with blood vessels, nutrients, and specialized cells that would support an early pregnancy.
If no fertilized egg implants, the body has no reason to maintain that thickened tissue. The upper two-thirds of the uterine lining breaks down in a piecemeal fashion, and the tissue exits through the cervix and vagina as menstrual fluid. This fluid is a mix of blood, mucus, and cells from the lining itself. Once shedding is complete, the surface repairs and the whole process starts over.
The Hormones That Trigger Bleeding
Two hormones drive the cycle: estrogen and progesterone. After an egg is released from the ovary (ovulation), progesterone rises sharply to keep the uterine lining thick and stable. If the egg isn’t fertilized within about two weeks, progesterone and estrogen both drop rapidly. That sudden hormone withdrawal is the direct trigger for menstruation. The lining can no longer sustain itself without hormonal support, so it breaks down.
When pregnancy does occur, the early embryo produces a hormone that keeps progesterone levels high. This is why pregnant women don’t get periods: the signal to shed the lining never arrives.
The Four Phases of the Cycle
A full menstrual cycle averages about 28 days, though anywhere from 21 to 35 days is common. It moves through four phases:
- Menstruation (days 1 to 3–7): The lining sheds and exits as a period.
- Follicular phase (days 1 to about 14): Overlapping with menstruation at the start, rising estrogen levels prompt the uterine lining to begin rebuilding. Meanwhile, small fluid-filled sacs (follicles) develop on the ovaries, each containing an egg.
- Ovulation (around day 14): One mature egg is released from the ovary into the fallopian tube, where it can potentially be fertilized.
- Luteal phase (days 15 to 28): Progesterone rises to keep the lining thick. If no pregnancy occurs, hormone levels crash and the cycle resets with another period.
Why Periods Cause Cramps
To push the old lining out, the uterus contracts. These contractions are driven by chemical messengers called prostaglandins, which increase in the uterus as the lining breaks down. Higher prostaglandin levels mean stronger contractions, which squeeze blood vessels in the uterine wall and temporarily reduce blood flow to the muscle. That combination of contraction and reduced blood flow is what produces the cramping sensation many people feel in the lower abdomen or back.
This is also why over-the-counter pain relievers like ibuprofen tend to help. They work by blocking prostaglandin production, which reduces both the intensity of the contractions and the pain that follows.
When Periods Typically Start
For girls born in the 2000s, the average age at first period is about 11.9 years, according to a large U.S. study published in JAMA Network Open. Starting before age 11 is considered early, before age 9 is very early, and starting at 16 or later is considered late. All of these are relatively uncommon.
The first period doesn’t arrive out of nowhere. Breast development is typically the earliest visible sign of puberty, appearing on average about 2 to 2.5 years before menstruation begins. Pubic and underarm hair usually follow about 1 to 1.5 years after breast development starts. Many girls also notice a white or clear vaginal discharge in the months leading up to their first period. These physical changes are reliable signals that menstruation is approaching.
Why Early Periods Are Often Irregular
It’s common for periods to be unpredictable for the first one to three years. The hormonal communication system between the brain and the ovaries takes time to mature. In the early months, the signals that coordinate ovulation and hormone production aren’t yet running on a tight schedule. Studies of adolescent girls show that while most do ovulate during their first few postmenarchal years, the hormonal patterns behind each cycle are often incomplete or inconsistent.
This means cycles might be 21 days one month and 40 the next, or periods might skip a month entirely. This gradually stabilizes as the hormonal system matures, typically within two to three years of the first period.
What Counts as Normal vs. Heavy Bleeding
Normal menstrual blood loss is under 60 mL per cycle. That can be hard to visualize, so a practical way to gauge it: if you’re soaking through a pad or tampon every hour for several hours in a row, that’s heavier than typical. Bleeding that lasts longer than 7 days or includes large blood clots are both independent markers of heavy menstrual bleeding, which affects a significant number of people and is worth bringing up with a healthcare provider.
Clinically, blood loss over 80 mL per cycle is the threshold for heavy menstrual bleeding. Some people naturally bleed more than others, and flow often varies from cycle to cycle. The first and second days tend to be heaviest, with lighter flow toward the end.