A menstrual cycle is a roughly monthly sequence of hormonal shifts that prepare the body for a possible pregnancy, then reset when pregnancy doesn’t occur. The average cycle lasts about 28 days, but anywhere from 21 to 35 days is normal for adults. The whole process is driven by four hormones working in a feedback loop, and it plays out across four distinct phases.
The Four Phases of the Cycle
Every cycle moves through the same sequence: the menstrual phase, the follicular phase, the ovulatory phase, and the luteal phase. These phases overlap slightly, and their timing can shift from cycle to cycle, but the pattern stays the same.
The menstrual phase (days 1 through 5, roughly) is the period itself. The uterine lining sheds because the body didn’t receive a pregnancy signal during the previous cycle. Bleeding typically lasts 3 to 7 days. Day 1 of bleeding is considered day 1 of the entire cycle.
The follicular phase starts on that same day 1 and runs until ovulation, around day 14. During this stretch, the brain’s pituitary gland releases a hormone that stimulates the ovaries to develop small fluid-filled sacs called follicles. Each follicle contains an immature egg, but usually only one follicle becomes dominant and matures fully. As the follicle grows, it pumps out rising levels of estrogen, which signals the uterus to start rebuilding its lining. By about day 7, estrogen levels climb significantly, and the uterine lining thickens to around 12 to 13 millimeters before ovulation.
The ovulatory phase (around days 14 to 16) is the shortest phase. When estrogen reaches a high enough level, usually around day 13, it triggers a sudden surge of luteinizing hormone (LH). Ovulation happens 28 to 36 hours after that LH surge begins, releasing the mature egg from its follicle into the fallopian tube. The egg survives for less than 24 hours after release. That narrow window is the only time fertilization can happen, though sperm can survive in the reproductive tract for several days beforehand.
The luteal phase (roughly days 15 through 28) fills the second half of the cycle. After the egg leaves, the empty follicle transforms into a temporary structure called the corpus luteum, a yellowish mass of cells that produces progesterone. This hormone further thickens and enriches the uterine lining, which can reach 16 to 18 millimeters, preparing it to support a fertilized egg. If no pregnancy occurs, the corpus luteum breaks down about 10 days after ovulation. Progesterone drops, the lining can no longer sustain itself, and menstruation begins again.
How the Hormones Work Together
Four hormones orchestrate the cycle through a feedback loop. Two come from the pituitary gland in the brain: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Two come primarily from the ovaries: estrogen and progesterone. The pituitary gland actually begins secreting FSH and LH before bleeding even starts, kicking off the next round of follicle development while the current cycle is still winding down.
FSH does what its name suggests: it stimulates follicle growth and causes estrogen to rise. As estrogen climbs through the first half of the cycle, it has two jobs. It rebuilds the uterine lining, and it communicates back to the brain. When estrogen hits a critical threshold, the pituitary responds with the LH surge that triggers ovulation. After ovulation, the system shifts. The corpus luteum takes over hormone production, flooding the body with progesterone. If there’s no pregnancy, the corpus luteum degrades, progesterone plummets, and the drop in hormones is the direct signal that causes the lining to shed. The brain senses those low hormone levels and ramps up FSH again, and the loop restarts.
If pregnancy does occur, the corpus luteum keeps producing progesterone for about 10 weeks, until the placenta takes over that role.
Why Periods Cause Cramps and Pain
The uterine lining doesn’t just passively fall away. Your body produces chemical signals called prostaglandins that cause the uterine muscles to contract, physically squeezing the lining out. These contractions are necessary for menstruation to happen, but the intensity varies from person to person. Higher levels of prostaglandins cause stronger contractions, which translates to more painful cramps. Excess prostaglandins can also contribute to heavier bleeding. This is why anti-inflammatory pain relievers, which reduce prostaglandin production, tend to be effective for period cramps.
What Counts as a Normal Cycle
A “normal” cycle has more range than most people expect. For adults, cycles anywhere from 21 to 35 days are considered healthy. Bleeding can last 2 to 7 days. Your cycle might be predictably regular or somewhat irregular from month to month, and both can be fine.
For adolescents, the range is even wider. In the first year after a period starts (the median age for that is about 12 and a half), cycles average around 32 days but can stretch up to 45 days. It takes a few years for the hormonal feedback loop to settle into a consistent rhythm. Adolescents going more than 3 months between periods should be evaluated, and anyone who hasn’t had a first period by age 15 should talk to a healthcare provider.
Using 3 to 6 pads or tampons per day is typical. Needing to change menstrual products every 1 to 2 hours is considered excessive, especially when combined with bleeding that lasts more than 7 days.
Your Cycle as a Health Signal
The American College of Obstetricians and Gynecologists recommends treating the menstrual cycle as a vital sign, on par with blood pressure, heart rate, and respiratory rate. The logic is straightforward: the cycle depends on a complex hormonal system, so disruptions to that system often show up as changes in your period before they show up anywhere else. Cycles that suddenly become much longer, shorter, heavier, or more painful can point to thyroid issues, hormonal imbalances, stress responses, nutritional deficiencies, or reproductive conditions.
Tracking your cycle doesn’t require anything complicated. Note the start date, how long bleeding lasts, and any symptoms that stand out. Over a few months, patterns emerge that make it easier to spot when something genuinely shifts versus normal variation. That baseline becomes useful information if you ever need to discuss changes with a provider.