The menstrual cycle is a roughly monthly process in which your body prepares for pregnancy, then resets if pregnancy doesn’t happen. A typical adult cycle lasts 21 to 34 days, counted from the first day of one period to the first day of the next. The whole process is driven by four hormones that rise and fall in a specific sequence, triggering changes in your ovaries and uterus at each stage.
The Four Phases at a Glance
The cycle has four overlapping phases: menstruation, the follicular phase, ovulation, and the luteal phase. Each one is defined by a shift in hormone levels that tells your body what to do next. In a textbook 28-day cycle, menstruation covers roughly days 1 through 5, the follicular phase runs from day 1 through about day 13, ovulation happens around day 14, and the luteal phase fills the remaining two weeks. In reality, the timing varies from person to person and even cycle to cycle.
Menstruation: The Reset
Your period marks day 1 of a new cycle. It happens because progesterone, the hormone that was sustaining the uterine lining, drops sharply when no fertilized egg implants. That progesterone withdrawal is the direct trigger for the lining to break down and shed. Most periods last about 4 to 5 days, and the total blood loss is small, roughly 2 to 3 tablespoons. Bleeding that lasts longer than 7 days, or that soaks through a pad or tampon in under 2 hours, is considered heavy.
The Follicular Phase: Building Up
The follicular phase starts on the same day as your period and continues until ovulation. Early on, your pituitary gland (a small gland at the base of your brain) releases follicle-stimulating hormone, or FSH. FSH signals your ovaries to start developing several fluid-filled sacs called follicles, each containing an immature egg.
Over the next week or so, one follicle outpaces the others and becomes dominant. That dominant follicle produces increasing amounts of estrogen, which does two important things. First, it thickens the lining of your uterus so it could support a fertilized egg. Second, once estrogen climbs high enough, it signals the brain to trigger the next phase: ovulation. The follicular phase is the most variable part of the cycle, which is why total cycle length differs so much between people.
Ovulation: Releasing the Egg
When estrogen reaches a critical level, the pituitary gland responds with a sudden burst of luteinizing hormone, known as the LH surge. Ovulation follows 8 to 20 hours after that LH peak. The dominant follicle ruptures and releases a mature egg into the fallopian tube, where it can be fertilized for roughly 12 to 24 hours. In a 28-day cycle, this typically happens around day 14, but it can shift earlier or later depending on how long your follicular phase runs.
This is the fertile window. Because sperm can survive in the reproductive tract for up to five days, pregnancy is possible from intercourse that happens several days before ovulation through about a day after it.
The Luteal Phase: Waiting for a Signal
After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. This structure pumps out progesterone, which stabilizes and enriches the uterine lining, making it a suitable environment for an embryo to implant. Progesterone levels peak about 6 to 8 days after ovulation.
Unlike the follicular phase, the luteal phase is relatively fixed in length, typically lasting 12 to 14 days (though it can range from 11 to 17 days). If a fertilized egg implants, it sends a hormonal signal that keeps the corpus luteum alive, and progesterone stays high. If implantation doesn’t happen, the corpus luteum breaks down, progesterone plummets, and the lining sheds. Your period starts, and the cycle begins again.
What Counts as a Normal Cycle
For adults, a cycle length between 21 and 34 days is considered normal. Teenagers operate on a wider range. In the first few years after a first period, cycles of 21 to 45 days are typical because the hormonal communication system between the brain and ovaries is still maturing. About 90% of adolescent cycles fall within that range. By the third year after a first period, 60 to 80% of cycles settle into the adult 21-to-34-day window.
Variation of a few days from one cycle to the next is completely normal. Your cycle length is not a fixed number; it’s a range. Stress, travel, illness, sleep changes, and weight fluctuations can all nudge a cycle shorter or longer without signaling a problem.
When Cycles Become Irregular
Some patterns are worth paying attention to. Cycles that consistently come more often than every 21 days or less often than every 45 days fall outside the expected range. Going 90 or more days between periods, even once, is also considered abnormal. The same goes for periods lasting longer than 7 days or bleeding that’s heavy enough to require changing a pad or tampon every hour or two.
Several medical conditions can disrupt the cycle’s hormonal choreography. Polycystic ovary syndrome (PCOS) is one of the most common, often causing infrequent or skipped periods along with other hormonal symptoms. Thyroid disorders can speed up or slow down cycles because thyroid hormones interact with the reproductive system. Extreme exercise or very low body weight can suppress the hormonal signals from the brain that kick off each cycle, causing periods to stop altogether. Growths in the uterus, like polyps, can cause heavier or longer bleeding without necessarily affecting cycle timing.
How Cycles Change With Age
Your cycle doesn’t stay the same throughout your reproductive years. In perimenopause, which typically begins in the mid-to-late 40s but can start earlier, ovulation becomes less predictable. Periods may come closer together or further apart, and flow can swing between unusually light and unusually heavy. If your cycle length starts shifting by 7 or more days from what’s been normal for you, that’s often an early sign of perimenopause. Later in the transition, gaps of 60 days or more between periods are common. Once you’ve gone 12 consecutive months without a period, you’ve reached menopause, and the cycle stops entirely.