A menstrual period is the monthly shedding of the uterine lining, released from the body as bleeding through the vagina. It happens when an egg released that cycle isn’t fertilized, signaling the body to clear out the tissue it had built up in preparation for pregnancy. Most periods last two to seven days, and cycles typically repeat every 21 to 35 days.
What Happens Inside Your Body
Each month, the uterus builds a thick, blood-rich lining designed to support a fertilized egg. When pregnancy doesn’t occur, levels of two key hormones (estrogen and progesterone) drop sharply. Without those hormones sustaining it, the lining can no longer hold together. Blood vessels in the uterine wall constrict, cutting off blood flow to the surface layers. The tissue breaks down and is shed.
What comes out isn’t purely blood. Menstrual fluid is a mix of blood, cells from the uterine lining, vaginal secretions, and enzymes that help break down the tissue. The total volume lost during one period is usually about 60 milliliters, roughly 2.7 ounces. Losing more than 80 milliliters regularly is considered heavy bleeding.
The Four Phases of the Cycle
A menstrual cycle is more than just the days you bleed. It includes four distinct phases, each driven by shifting hormone levels.
Menstruation
This is the bleeding phase, lasting anywhere from two to eight days, with four to six days being the average. The uterine lining releases chemicals called prostaglandins, which cause the uterine muscles to contract and push out the tissue. These contractions are what you feel as cramps. Prostaglandin levels are highest on the first day of your period, which is why cramps tend to be worst at the start and ease as bleeding continues.
Follicular Phase
This phase overlaps with menstruation and continues after bleeding stops. It lasts roughly 10 to 16 days and is the most variable part of the cycle, which is why some people have shorter or longer cycles overall. During this time, a hormone called FSH stimulates the ovaries to develop a group of egg-containing follicles. One follicle becomes dominant and grows, producing rising levels of estrogen. That estrogen signals the uterus to start rebuilding its lining.
Ovulation
When estrogen reaches a critical threshold, it triggers a surge of another hormone called LH. About 10 to 12 hours after that surge peaks, the dominant follicle releases its egg into the fallopian tube. This is the fertile window. The entire ovulation event is brief, but the egg can survive for about 12 to 24 hours after release.
Luteal Phase
After the egg is released, the empty follicle transforms into a temporary structure that pumps out progesterone. This hormone thickens and stabilizes the uterine lining, making it ready to receive a fertilized egg. The luteal phase is remarkably consistent across most people, lasting about 14 days. Progesterone and estrogen peak around eight to nine days after ovulation. If the egg isn’t fertilized, that temporary structure breaks down, hormone levels plummet, and the cycle starts over with menstruation.
When Periods Typically Start and Stop
The first period, called menarche, happens during puberty. CDC data from 2013 to 2017 shows that half of girls in the United States reached their first period by age 11 years and 10 months. By age 14, about 90% had started menstruating. That median age has shifted slightly earlier over the decades, dropping from 12.1 years in 1995 to 11.9 in 2013 to 2017.
Periods continue until menopause, which generally occurs between ages 45 and 55. In the years leading up to menopause, cycles often become irregular as hormone production fluctuates, with periods arriving closer together or further apart before stopping entirely.
Why Periods Hurt
Cramping during a period is caused by prostaglandins, the same chemicals that trigger the uterine contractions needed to shed the lining. Higher prostaglandin levels mean stronger contractions and more pain. This type of period pain is called primary dysmenorrhea, and it’s extremely common. The pain typically centers in the lower abdomen and can radiate to the lower back and thighs.
Some people experience period pain caused by an underlying condition affecting the reproductive organs, such as endometriosis or fibroids. This is called secondary dysmenorrhea and tends to get worse over time rather than better, which distinguishes it from the ordinary cramping most people experience.
What Counts as a Normal Period
Normal varies quite a bit from person to person, but there are general ranges. A cycle length of 21 to 35 days is considered typical, counting from the first day of one period to the first day of the next. Bleeding that lasts two to seven days falls within the expected range. Some variation between cycles is normal too, but a gap of more than nine days between your shortest and longest cycles (for example, 28 days one month and 38 the next) is worth paying attention to.
Signs of Irregular Bleeding
Certain patterns fall outside the normal range and are worth investigating. These include cycles shorter than 21 days or longer than 35 days, missing three or more periods in a row (when you’re not pregnant, breastfeeding, or approaching menopause), and bleeding that lasts longer than seven days. Spotting between periods or after sex can also signal something that needs attention.
Heavy bleeding has a more specific threshold than most people realize. If you’re soaking through a pad or tampon in an hour, or regularly losing more than about 80 milliliters per cycle, that crosses into heavy menstrual bleeding. Periods accompanied by severe pain, nausea, or vomiting also fall outside what’s considered typical and are worth discussing with a healthcare provider.