The Menstrual Cycle

Your body runs a monthly rehearsal for pregnancy, and when the show does not go on, it cleans up the stage.

By Greg Goodwin · Published October 17, 2026 · Updated October 24, 2026

Cheat Sheet

Counting rule: day 1 is the first day of bleeding, and the cycle ends the day before the next period starts.
Length: the average is about 28 days, and ACOG describes cycles of 21 to 45 days as normal for young people; individual variation is expected.
Bleeding: ACOG says 2 to 7 days is normal; MedlinePlus says a period usually lasts 3 to 7 days.
First period: MedlinePlus says most start between ages 9 and 15; ACOG says most start at 12 to 13.
Settling in: it can take 6 years or more after the first period for cycles to become regular.
End point: periods usually continue until menopause, which most often happens between ages 45 and 55.

The 60-Second Version

The menstrual cycle is the monthly sequence of hormone-driven changes that prepares the body for a possible pregnancy. Signals from the brain prompt the ovaries to make hormones, and those hormones thicken the lining of the uterus with extra blood and tissue. Partway through the cycle, an ovary releases an egg in a process called ovulation, and the egg travels down a fallopian tube toward the uterus. If it is not fertilized, hormone levels shift, the lining breaks down, and it leaves the body through the vagina. That bleeding is the period. Doctors count the cycle from the first day of one period to the first day of the next. Plenty of people fall near 28 days, but shorter and longer cycles can be perfectly normal, and cycles vary from person to person and from month to month. This is general information, not medical advice.

The Long Version

Counting a Cycle

A menstrual cycle is measured from the first day of bleeding in one month to the first day of bleeding in the next. The average length is about 28 days, but the American College of Obstetricians and Gynecologists says cycles from 21 to 45 days are also normal for young people, and MedlinePlus notes that cycles can be shorter or longer and still be normal. Sources give slightly different ranges, so treat any single number as a typical value rather than a rule. Tracking your own pattern, by marking the first day of bleeding on a calendar or in an app, is the easiest way to learn what is normal for you and to notice changes.

The Hormone Relay

At puberty, the brain begins signaling the body to produce hormones. The ovaries make estrogen, progesterone, and testosterone, and hormones in this relay cause the uterine lining to thicken each month. One ovary then releases an egg. The egg moves along a fallopian tube toward the uterus. If it meets sperm and is fertilized, pregnancy can begin. If not, the lining is shed as menstrual blood and tissue, and the cycle starts over. The details of exactly how hormone levels rise and fall differ between people, which is one reason cycle timing is so variable.

What a Period Feels Like

Bleeding can be light or heavy. ACOG describes anywhere from 2 to 7 days as normal, and MedlinePlus says a period usually lasts three to seven days. Along with bleeding, many people have cramping in the lower abdomen or pelvis, lower back pain, bloating, sore breasts, food cravings, mood swings, headaches, or fatigue. Premenstrual syndrome, or PMS, is a group of physical and emotional symptoms that can appear in the days before a period and usually fade once bleeding starts. People manage bleeding with pads, tampons, menstrual cups, or period underwear, and changing or cleaning products regularly keeps things comfortable.

Variation Is Normal

Most girls get a first period somewhere between ages 9 and 15, with ACOG noting that 12 to 13 is most common and that periods often begin about two to three years after breast growth starts. The first few periods can be very light, and it may take six years or more for cycles to become regular. Hormonal shifts after childbirth or during breastfeeding can change periods too. Periods usually continue until menopause, which most often occurs between ages 45 and 55. Because ranges are broad, a cycle that differs from a friend's or from an app's default is usually not a sign of a problem by itself.

When to Check In With a Clinician

Menstrual irregularities include missed periods, irregular periods, very heavy bleeding, and painful periods. Terms you might hear include amenorrhea (no periods), oligomenorrhea (infrequent periods), menorrhagia (very heavy bleeding), and dysmenorrhea (painful periods). MedlinePlus describes prolonged bleeding as periods that regularly last longer than 8 days. ACOG's guidance for young people suggests seeing a doctor if no period has started by age 15, if periods stop for more than 3 months, if bleeding soaks through a pad or tampon every 1 to 2 hours, or if cycles come more often than every 21 days or less often than every 45 days. Severe pain, dizziness, or a racing pulse also deserve prompt attention. For any personal concern, a clinician can give advice that general information cannot.

Why People Care

Understanding the cycle demystifies a monthly experience shared by a huge number of people, and it helps tell normal variation from signs worth raising with a clinician. Knowing your own pattern also makes conversations with a doctor faster and more useful.

Glossary

Ovulation: The point in the cycle when an ovary releases an egg.
Uterine lining: The tissue inside the uterus that thickens each cycle and is shed as menstrual blood if pregnancy does not occur.
Estrogen and progesterone: Hormones made by the ovaries that help drive the cycle.
Amenorrhea: The absence of menstrual periods in someone of reproductive age.
Dysmenorrhea: Painful periods, which can include severe cramps.

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