It's not one event — it's a loop
The menstrual cycle isn't the period. The period is one of four phases in a hormonal loop that runs roughly every 21 to 35 days, coordinated by the brain and the ovaries talking to each other.
Day 1 is the first day of bleeding. From there, four phases play out: menstruation, the follicular phase, ovulation, and the luteal phase. Then the loop starts again.
The four phases
- Menstruation (days 1–5, roughly)
- The uterus sheds the endometrial lining built up last cycle. Estrogen and progesterone are at their lowest.
- Follicular (days 1–13, overlaps with the period)
- The pituitary releases FSH. Ovaries ripen 5–20 follicles. Estrogen climbs. Energy and mood usually rise with it.
- Ovulation (around day 14 in a 28-day cycle)
- A surge of LH releases one mature egg. It lives for 12–24 hours. This is the fertile window.
- Luteal (days 15–28)
- The empty follicle becomes the corpus luteum and pumps out progesterone. If no pregnancy, progesterone crashes, the lining sheds, and you're back at day 1.
The brain is running the show
The hypothalamus in your brain releases GnRH, which tells the pituitary to release FSH and LH, which tell the ovaries to make estrogen and progesterone. This is the HPO axis — hypothalamic-pituitary-ovarian.
Sleep, stress, illness, weight change, and thyroid problems all reach into this axis. A skipped or late period is often the brain reading conditions as unsafe and quietly pressing pause.
Self-check
Quick check
1. Which phase is driven mainly by progesterone?
2. What does the LH surge do?