Menstrual cycle: definition and meaning
The menstrual cycle is the roughly monthly series of hormone-driven changes in the ovaries and uterus that prepares the body for a possible pregnancy. It runs from the first day of one period to the day before the next. A typical adult cycle lasts 21 to 35 days, with 28 days being an average, not a rule.
The four phases
| Phase | Roughly when | What happens |
|---|---|---|
| Menstrual | Days 1 to 5 | The uterine lining sheds as a period |
| Follicular | Day 1 to ovulation | Follicles grow, estrogen rises, the lining rebuilds |
| Ovulation | About 14 days before the next period | An egg is released |
| Luteal | After ovulation to the next period | Progesterone rises, then falls if no pregnancy |
The follicular and menstrual phases overlap, since the follicular phase starts on day one of bleeding. Cycle length differences mostly come from the follicular phase; the luteal phase is usually 12 to 14 days.
Who runs the cycle
The brain and ovaries talk to each other. The hypothalamus releases GnRH, the pituitary answers with FSH and LH, and the ovaries respond with estrogen and progesterone, which in turn feed back to the brain. Stress, illness, weight changes and exercise can all nudge this loop.
Your cycle and your mood
Read the full guide to menstrual cycle phases.
Frequently asked questions
Is a 28-day cycle normal?
It's an average. Anything from 21 to 35 days is normal for adults, and teens can have cycles up to 45 days.
Do I ovulate on day 14?
Only if your cycle is about 28 days. Ovulation usually happens about 12 to 14 days before your next period, so it moves with your cycle length.
Is the menstrual cycle the same every month?
No. A few days of variation is normal, and a big study of over 600,000 cycles found most people's cycles vary somewhat.
Sources
- Office on Women's Health: Your menstrual cycle
- Cleveland Clinic: Menstrual Cycle
- Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med (2019)
This page is for information only and isn’t medical advice. If something worries you, please talk to a doctor.
Keep reading
The 4 phases of the menstrual cycle, in colorThe 4 phases of the menstrual cycle explained simply: what your hormones do in each, how long each lasts, and how mood and energy usually change.
Menstrual phase: definition and meaningThe menstrual phase is the part of your cycle when you have your period, usually days 1 to 5. What happens to hormones, energy and mood during it.
Luteal phase: definition and meaningThe luteal phase is the part of your cycle from ovulation to your next period, usually 12 to 14 days. Hormones, mood, PMS and why it matters.
Cycle length: definition and meaningCycle length is the number of days from the first day of one period to the day before the next. What's normal, how to measure it, and why it varies.What phase of my cycle am I in? Free phase finderWhat phase of my cycle am I in? Enter your last period to see your cycle day, today's phase and how people often feel in it. Private and instant.