How the menstrual cycle works
The menstrual cycle is a monthly loop coordinated by the brain and ovaries. It begins on the first day of your period and runs until the day before your next one. Across it, four hormones rise and fall in a predictable pattern: FSH gets follicles growing, estrogen rebuilds the uterine lining and peaks before ovulation, a surge of LH releases the egg, and progesterone prepares the lining for a possible pregnancy. If pregnancy doesn't happen, hormones fall and the lining sheds — a new period, and a new cycle.
Understanding this pattern helps make sense of a lot of midlife changes. As you move into perimenopause, ovulation becomes less regular and these hormone curves get erratic, which is why periods become irregular. Our guide to the phases of the menstrual cycle goes deeper, and if your cycles are very heavy, painful, or unpredictable, see heavy periods and spotting between periods. To estimate your fertile window, try the period & ovulation tracker.
Frequently asked questions
What are the four phases of the menstrual cycle?
The cycle has four phases: menstruation (the period, days 1–5), the follicular phase (days 1–13, when a follicle matures and estrogen rises), ovulation (around day 14, when an egg is released), and the luteal phase (days 15–28, when progesterone rises and then falls before the next period).
Which hormones control the menstrual cycle?
Four hormones do most of the work: FSH recruits follicles early; estrogen rises through the follicular phase and peaks before ovulation; a surge of LH triggers ovulation; and progesterone rises through the luteal phase to prepare the uterine lining. If there's no pregnancy, estrogen and progesterone fall and the next period begins.
When am I most fertile?
The fertile window is the roughly six days ending on ovulation day, because sperm can survive several days and the egg lives about 12–24 hours. Ovulation is often near day 14 of a 28-day cycle, but it varies a lot between people and even between cycles, so timing is only an estimate.
Is a 28-day cycle normal?
A 28-day cycle is the textbook average, but a normal cycle can run anywhere from about 21 to 35 days, and length can vary month to month. What matters more than hitting exactly 28 days is that your own pattern is fairly consistent and not causing problems.
Can this tool tell me where I am in my cycle?
No. This explorer is for education only. It shows a typical, illustrative pattern — it can't track your cycle, confirm ovulation, or be used for contraception or to plan or avoid pregnancy. For that, talk with a clinician about appropriate methods.