The menstrual cycle is confusing because most textbooks describe it as a single 28-day timeline, when it is actually two cycles happening simultaneously — one in the ovary and one in the uterus — coordinated by four hormones. Once you align the ovarian cycle and the uterine cycle on the same calendar, hormone by hormone, the whole topic falls into a single chart you can sketch from memory. This walkthrough builds that chart in order.
Two Cycles, Same 28 Days
The "menstrual cycle" is shorthand for two parallel events.
- The ovarian cycle is what happens in the ovary: a follicle matures, an egg is ovulated, and the leftover follicle becomes the corpus luteum.
- The uterine cycle is what happens in the uterus: the endometrium sheds, rebuilds, and prepares to receive an embryo.
Both run on the same clock, which by convention starts on day 1 — the first day of bleeding. A typical cycle is 28 days, with ovulation around day 14, though normal cycles range from about 21 to 35 days.
Four hormones drive both cycles. Two come from the anterior pituitary: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Two come from the ovary itself: estrogen (mainly from the maturing follicle) and progesterone (mainly from the corpus luteum). The brain releases gonadotropin-releasing hormone (GnRH) upstream of all of them.
Days 1–5: Menstrual Phase
Day 1 is the first day of bleeding. The endometrium — the thick lining of the uterus that built up the previous month — sheds. This is the uterine event.
At this point, all four hormones are at their lowest point in the cycle. Progesterone in particular has crashed (that crash is what caused the lining to shed). Low estrogen and progesterone release the brain from negative feedback, so the hypothalamus begins releasing GnRH again, which nudges the pituitary to release FSH.
In the ovary, FSH is now starting to stimulate a fresh cohort of follicles.
Days 6–14: Follicular Phase (Ovarian) and Proliferative Phase (Uterine)
Ovarian side. FSH stimulates several follicles to grow, each containing an immature egg. Within a few days, one becomes dominant and the others regress. The dominant follicle secretes increasing amounts of estrogen as it matures.
Uterine side. Estrogen from the growing follicle drives the proliferative phase in the uterus — the endometrium rebuilds, thickens, and grows new blood vessels and glands. The lining goes from a thin band a few millimeters thick to roughly 8–12 mm by the end of the phase.
The feedback story is the most important part. For most of the cycle, estrogen feeds back negatively on the hypothalamus and pituitary, keeping its own levels in check. But late in the follicular phase, estrogen from the dominant follicle climbs high enough — and stays high long enough — that feedback briefly switches to positive. Suddenly more estrogen increases GnRH and pituitary output instead of suppressing it. The result is a sharp surge in LH (and a smaller bump in FSH).
Day 14: Ovulation
The LH surge triggers ovulation. About 24–36 hours after the surge begins, the dominant follicle ruptures and releases its egg into the fallopian tube. Some women feel this — a small one-sided pelvic twinge called mittelschmerz.
This is the only day in the cycle when a clean handoff between the two halves makes sense. Before day 14, the follicle has been the dominant hormonal source. After day 14, the remnant of that follicle takes over.
Days 15–28: Luteal Phase (Ovarian) and Secretory Phase (Uterine)
Ovarian side. The leftover follicle, now without its egg, transforms into the corpus luteum — a temporary endocrine structure. Under continued LH support, the corpus luteum secretes large amounts of progesterone plus some estrogen.
Uterine side. Progesterone drives the secretory phase. The endometrium does not grow much thicker, but it becomes glandular and vascular, ready to receive an embryo. Glands begin secreting nutrients; the lining is "ready and waiting."
Progesterone and estrogen now feed back negatively again on the hypothalamus and pituitary, suppressing FSH and LH. This blocks any new follicles from developing during the luteal phase — the body is committing to this one ovulation.
What happens next depends on whether fertilization occurred.
- If fertilization happens and the embryo implants around day 21, the developing placenta secretes human chorionic gonadotropin (hCG), which keeps the corpus luteum alive. Progesterone stays high. The endometrium is maintained. No bleeding. (hCG is what pregnancy tests detect.)
- If fertilization does not happen, the corpus luteum has a built-in lifespan of about 10–14 days. Around day 26 it begins to degenerate (becoming the corpus albicans). Progesterone and estrogen crash. Without progesterone, the endometrium cannot be maintained, blood vessels constrict, the lining dies, and on day 28 (or so) it sheds — and the next cycle starts at day 1.
The Hormones, Aligned in One Picture
If you were to sketch the four-hormone graph from memory, the shapes you want are:
- FSH — a small rise early in the follicular phase, a smaller bump at mid-cycle, then low.
- LH — quiet for most of the cycle, then a sharp tall spike at day 14, then low.
- Estrogen — rises through the follicular phase to a peak just before ovulation, drops briefly at ovulation, then rises again to a smaller second peak during the luteal phase, then crashes.
- Progesterone — flat and low until ovulation, then climbs through the luteal phase to a clear peak around day 21–22, then crashes.
Two crashes, two peaks per hormone where relevant — that's most of the chart.
Getting Help
The brief switch from negative to positive feedback at mid-cycle is the same physiological mechanism covered in negative vs. positive feedback. The pituitary hormones controlling the cycle are part of the broader system explained in the endocrine system and its hormones. For more physiology walkthroughs, see the full set of Anatomy & Physiology study guides.
Conclusion
The menstrual cycle explained as two parallel cycles is much easier than as one 28-day calendar. In the ovary, FSH grows follicles, the dominant follicle pumps out estrogen, an LH surge ovulates the egg, and the corpus luteum makes progesterone for two weeks before retiring. In the uterus, estrogen rebuilds the endometrium, progesterone makes it secretory, and the loss of progesterone sheds it. Align both cycles and the four hormones on day 1 to day 28 and the whole topic fits on one chart.
