The Menstrual Cycle: Four Hormones in a Loop
The menstrual cycle is a roughly 28-day conversation between the brain and the ovaries, run by just four hormones in a feedback loop. Watch FSH grow a follicle, estrogen thicken the lining and then trigger a dramatic LH surge, the egg burst out at ovulation, and progesterone take over — until, with no pregnancy, everything crashes and the lining sheds. Press play and finally see how the pieces fit.
Try this: start on The LH surge & ovulation and watch the green estrogen line peak first — then, a beat later, the red LH line spikes and the egg pops. Then flip on On the pill and see the whole loop go flat.
Live cycle readout
What's happening
The curve shapes and the animation are a schematic model of a textbook ~28-day cycle — real cycles vary in length and height. The units in the readouts are realistic ranges (estradiol tens–hundreds of pg/mL, mid-luteal progesterone often 10–20 ng/mL, a mid-cycle LH surge, a ~0.5 °F temperature rise), shown to give a feel for scale rather than as measurements from any one person.
The Science in Plain Language
Four hormones, one loop
Your menstrual cycle is not run by the ovaries alone. It is a feedback loop between the brain and the ovaries, using four chemical messengers. The hypothalamus (deep in the brain) releases little pulses of GnRH (gonadotropin-releasing hormone). GnRH tells the pituitary gland just below it to secrete two hormones into the blood: FSH (follicle-stimulating hormone) and LH (luteinizing hormone). Those two travel to the ovaries, where they control the release of the two ovarian hormones you have probably heard of — estrogen and progesterone. Estrogen and progesterone then travel back to the brain to turn the whole thing up or down. That round trip — brain to ovary to brain — is the loop, and it takes about 28 days.
The follicular phase (about days 1–14): FSH grows a follicle
At the start of the cycle all four hormones are low. That low state is actually the trigger to begin: with nothing holding it back, the pituitary nudges FSH up, and FSH does exactly what its name says — it stimulates a batch of follicles (tiny fluid-filled sacs, each holding one immature egg) to start growing. Usually one becomes the dominant follicle and the rest fade. As it grows — watch it swell in the ovary panel — its cells pump out rising amounts of estrogen. Estrogen has a job in the uterus: it rebuilds and thickens the endometrium (the lining), taking it from a few millimeters up toward 7–8 mm, laying down blood supply for a possible pregnancy. This is why it is also called the proliferative phase.
The estrogen twist: positive feedback and the LH surge
Here is the single most surprising move in the whole cycle. For most of the month, estrogen tells the brain to calm down (negative feedback) — which is why rising estrogen actually lets FSH dip mid-cycle. But when estrogen climbs above a high threshold and stays there for a day or two, the feedback flips: high estrogen now tells the pituitary to fire. The pituitary answers with an enormous, brief spike of LH — the LH surge, the tallest peak on the whole chart. This flip from “quiet down” to “go” is one of the only examples of positive feedback in human physiology, and it is the hinge the entire cycle turns on. Notice the order on the graph: the green estrogen line peaks first, and the red LH spike follows. Estrogen is the cause; the surge is the effect.
Ovulation (around day 14): the egg is released
Roughly 24–36 hours after the LH surge begins, the surge does its work: it bursts the dominant follicle and the mature egg is swept out into the fallopian tube. That is ovulation — the one day (really a window of about 12–24 hours for the egg) when pregnancy is possible, though sperm can wait a few days. This is exactly what an ovulation-predictor kit (OPK) is detecting: those urine test strips turn positive when they sense the LH surge, giving you roughly a day of warning before the egg drops.
The luteal phase (about days 15–28): progesterone takes over
The follicle does not just vanish after releasing its egg. The emptied sac collapses and transforms into a temporary gland called the corpus luteum (Latin for “yellow body” — watch the yellow structure appear in the ovary). Its main product is progesterone, the hormone of the second half. Progesterone matures the thickened lining into a nourishing, glandular bed ready to receive an embryo (the secretory phase), and it quietly nudges your basal body temperature up by about 0.5 °F (0.3 °C) — the small, sustained rise that fertility-tracking apps look for to confirm that ovulation has already happened. Progesterone also clamps the brakes back on: it holds FSH and LH low, so no new follicle matures while the body waits to see if a pregnancy has begun.
No pregnancy: the crash, the period, and PMS
The corpus luteum is on a timer. If no embryo implants and sends a rescue signal (the pregnancy hormone hCG), the corpus luteum dies after about 12–14 days. When it dies, its hormones stop — estrogen and progesterone both crash. The lining has been depending on that hormone support, so without it the endometrium breaks down and sheds: that shedding, and the blood that goes with it, is your period. That same steep progesterone drop in the days before bleeding is the leading suspect behind PMS (premenstrual syndrome) — the mood changes, bloating, and irritability many people feel late in the luteal phase. And crucially, the crash is also the reset: with hormones low again, the brain is finally free to raise FSH, and the loop starts over. The end of one cycle is the beginning of the next.
The pill: borrowing the loop's own brakes
The combined birth-control pill is clever precisely because it uses the loop against itself. Each active pill delivers a steady dose of estrogen and a progestin (a progesterone-like hormone). Because those levels stay flat and never crash, the brain reads them as “the ovary is already handled” and keeps FSH and LH suppressed all month. No FSH means no follicle ripens; flat hormones mean no LH surge, so no egg is ever released. Turn on the pill scenario and you will see the whole four-hormone dance flatten out. The bleeding you get during the placebo (sugar-pill) week is not a true period — it is a withdrawal bleed from briefly stopping the hormones, and it is usually lighter than a natural period because the lining was kept thin.
PCOS: when the loop gets stuck
Polycystic ovary syndrome (PCOS) is one of the most common ways the loop goes wrong. In PCOS the signaling is often skewed — LH tends to run high relative to FSH, and no clean estrogen peak builds — so the LH surge never fires and ovulation is missed. Follicles start growing but stall as many small cysts (the “polycystic” look) instead of one dominant follicle bursting. Without ovulation there is no corpus luteum, so progesterone stays low and flat — which is why cycles become irregular or disappear, and why the basal temperature stays “monophasic” (no mid-cycle rise). Because estrogen keeps building the lining but progesterone never arrives to mature and then shed it on schedule, the lining can thicken unevenly. Flip on the PCOS toggle to watch the surge disappear and the loop stall.
What the numbers actually mean, and a myth to drop
A few practical anchors. A progesterone blood test above about 3 ng/mL in the second half of the cycle (often drawn around “day 21”) is the classic confirmation that ovulation happened — mid-luteal levels commonly run 10–20 ng/mL. Estradiol climbs from tens of pg/mL early to a couple hundred at its pre-ovulation peak. And here is the myth worth dropping: ovulation does not always fall on day 14, and a “normal” cycle is not always 28 days. Anything from about 21 to 35 days is typical. What is remarkably stable is the luteal phase — it stays close to 14 days for almost everyone. It is the follicular phase that stretches or shrinks, so a longer cycle usually means it took longer to ovulate, not that the second half ran long. That is exactly why tracking the LH surge or the temperature shift beats counting to 14 on a calendar.