The menstrual cycle unfolds as a chain reaction rather than a single switch: the hypothalamus releases GnRH, the pituitary responds with LH and FSH, the ovaries produce estrogen and progesterone, and bleeding actually begins when both estrogen and progesterone drop sharply after the corpus luteum breaks down at the end of an unfertilized cycle. That final withdrawal, not the hormone buildup that preceded it, is the direct trigger for menstrual bleeding.
This detailed guide walks through the hormonal relay behind menstruation, starting with GnRH in the brain and following the rise and fall of LH, FSH, estrogen, and progesterone that ultimately triggers bleeding.
The Hormonal Relay Race That Drives Your Cycle
Picture four stations passing a baton every month. The hypothalamus hands off to the pituitary gland, which signals the ovaries, which prepare the uterus. Each station releases a chemical messenger that triggers the next, and the cycle repeats roughly every 21 to 35 days. No single hormone acts alone, so each step sets the stage for what comes next in the hormonal changes responsible for menstruation.
The Four Key Players
The hypothalamus sits at the base of your brain and acts as the starting line. The pituitary gland, a pea-sized structure just below it, receives the hypothalamus’s signal and sends two gonadotropin hormones into your bloodstream. The ovaries respond by producing estrogen and progesterone, and the uterus reacts to those ovarian hormones by either building or shedding its inner lining, called the endometrium.
Why a Drop Matters More Than a Rise
Hormone levels rise and fall across the cycle, but the period starts because of a fall. A buildup of estrogen thickens the uterine lining during the first half of the cycle. Progesterone stabilizes that lining during the second half. When both hormones drop, the lining loses its support and sheds, which answers the common question of which hormone causes your period to start: it is the withdrawal of progesterone, with falling estrogen playing a supporting role.
How the Brain Starts the Chain With GnRH, LH, and FSH
Each month, your cycle restarts when the hypothalamus releases gonadotropin-releasing hormone, or GnRH, into nearby blood vessels. GnRH tells the pituitary gland to release two gonadotropin hormones: follicle-stimulating hormone, or FSH, and luteinizing hormone, or LH. These two signals open the follicular phase and set up ovulation roughly two weeks later, explaining how hormones control menstrual cycle signaling from the top down.
FSH Recruits the Follicles
Rising FSH travels to the ovaries, where it stimulates a group of follicles to grow. Each follicle contains an immature egg, and as the follicles mature, they begin producing estrogen. Rising estrogen levels feed back to your brain, gradually lowering FSH so usually one follicle takes the lead. That dominant follicle keeps growing and producing more estrogen, preparing the egg for release.
The LH Surge Triggers Ovulation
Once estrogen climbs high enough for long enough, your brain responds with a sharp spike in LH, called the LH surge. This surge, which lasts roughly 24 to 36 hours, causes the dominant follicle to rupture and release its egg, a moment called ovulation. The LH surge is the handoff from brain to ovaries and marks the transition from the follicular phase to the luteal phase.
That handoff only matters once the ovary has built up enough estrogen to respond, setting the stage for the rise and fall that follows.
Estrogen and Progesterone Build and Then Withdraw
After ovulation, the empty follicle transforms into a small temporary gland called the corpus luteum. The corpus luteum becomes the main source of progesterone during the second half of the cycle, while estrogen stays high as well, and together these two hormones prepare and stabilize the thickened uterine lining for a possible pregnancy.
The Luteal Phase Window
Most luteal phases run between 10 and 16 days, with 12 to 14 being the most common range. During this window, the corpus luteum steadily secretes progesterone and estrogen, keeping the endometrium thick, stable, and full of blood vessels. If a fertilized egg implants, the corpus luteum keeps producing progesterone to maintain the lining until the placenta takes over.
The Hormone Drop After No Pregnancy
When pregnancy does not occur, the corpus luteum breaks down around day 22 to 24 in a typical 28-day cycle. As it dissolves, both progesterone and estrogen levels fall sharply. The corpus luteum’s lifespan is fixed, which is why the luteal phase length stays fairly predictable from cycle to cycle, even when the follicular phase varies, and that predictable breakdown is the direct trigger for menstruation.
The Specific Hormone Drop That Actually Triggers Bleeding
Progesterone withdrawal is the event that destabilizes the endometrium and starts bleeding, which is the hormone drop to start period that matters most. Progesterone had been keeping the lining stable by keeping blood vessels firm and the tissue structure intact, so when progesterone drops, the lining’s structural support gives way, blood vessels in the endometrium constrict, and the outermost layers begin to break apart.
Estrogen’s Supporting Role
Falling estrogen contributes to the breakdown by removing the growth signal that thickened the lining in the first place. Without estrogen’s continued presence, the endometrial cells lose the molecular cues that maintain the tissue, so the result is a combined withdrawal effect: progesterone’s absence removes structural stability, and estrogen’s absence removes growth support, and the lining sheds.
Prostaglandins Finish the Job
As endometrial cells break down, they release prostaglandins, which are inflammatory lipid compounds that trigger uterine contractions. These contractions push menstrual blood and tissue out through the cervix and vagina. Prostaglandins also cause the cramps many people feel before bleeding, since higher prostaglandin levels typically mean stronger contractions and more discomfort.
Why Cycle Length and Symptom Timing Vary
The 28-day cycle taught in textbooks is real, but the healthy range runs from 21 to 35 days for adults, and can be slightly longer for teens. Cycle variation mostly comes from the follicular phase, which can stretch or shorten based on how quickly the brain-ovary signaling recruits a dominant follicle, while the luteal phase stays more consistent because the corpus luteum has a fixed lifespan.
Recognizable Pre-Period Signs
The hormonal changes in the late luteal phase create a predictable symptom pattern in the week before bleeding starts. Common signs include breast tenderness from falling estrogen and progesterone shifts, mood changes tied to shifting neurotransmitter sensitivity, fluid-related bloating, and food cravings linked to metabolic changes, while mild cramps often begin a day or two before bleeding as prostaglandins rise.
Why Individual Experience Varies
Different people have different hormone sensitivities, so one person’s “mild PMS” can feel like a storm to someone else. The absolute levels of progesterone and estrogen matter less than how strongly your body responds to them, because hormone receptor sensitivity, baseline neurotransmitter levels, stress, sleep, and nutrition shape how each phase feels from one person to the next.
When Hormonal Contraception Changes the Pattern
Hormonal contraceptives override the brain-ovary-uterus relay with steady outside hormone levels, and the exact mechanism depends on which type you use, since combined methods and progestin-only methods affect the cascade in different ways. The table below summarizes how each method changes your natural hormonal pattern.
| Method Type | How Hormones Are Delivered | Effect on the Cycle |
|---|---|---|
| Combined pills, patch, or vaginal ring | Steady estrogen and progestin across the cycle | Suppresses ovulation; the monthly bleed is a hormone-free interval bleed, not a true period |
| Progestin-only pill, implant, or hormonal IUD | Continuous low progestin without estrogen | Often thins the endometrium and may stop withdrawal bleeding entirely |
| Copper IUD | No hormones | Natural hormonal cascade stays intact; periods continue with potentially heavier bleeding |
| Depo-Provera injection | High progestin every three months | Often stops periods after the first year of use |
The Monthly Bleed on the Pill Is Not a True Period
The bleeding during the placebo week of combined pills happens because the hormone-free interval causes a short, controlled withdrawal of estrogen and progestin. The endometrium does shed a little, but it is typically thinner than in a natural cycle, and the bleed is usually lighter and shorter than a natural period, which is why clinicians call it a withdrawal bleed rather than a true menstrual period.
Why Some Bleeding Disappears
Progestin-only methods keep the endometrium thin and stable by mimicking the stabilizing effects of progesterone, which is why many users experience light bleeding or no bleeding at all. The lining never thickens enough to shed dramatically, and from a hormonal standpoint, the cycle’s natural peaks and valleys are flattened so the withdrawal that triggers bleeding never happens in the same way.
Because that withdrawal is chemically muted, anything unusual on the calendar becomes harder to interpret without a clearer sense of what to watch for.
Recognizing When a Hormonal Pattern Needs Attention
Cycle variation is normal, but certain patterns suggest the hormonal cascade may be disrupted at one of its stations. Tracking your cycle for two to three months gives a clearer picture than trying to remember symptoms in the moment, and if something feels consistently off, that pattern is worth discussing with a qualified healthcare professional.
Red-Flag Cycle Patterns
Watch for cycles that fall outside normal range: periods absent for three or more months in a row, periods lasting longer than seven days, or cycles consistently shorter than 21 days or longer than 35 days all warrant a medical evaluation. Bleeding that soaks through a pad or tampon every hour for several hours also falls outside typical range and deserves prompt attention.
Conditions That Disrupt the Cascade
Polycystic ovary syndrome (PCOS) disrupts ovulation by flooding the ovaries with androgens and disrupting normal follicle development, which can lead to skipped or absent periods because the LH surge never fires. Thyroid disorders shift brain signaling at the hypothalamus and pituitary level, changing how quickly GnRH pulses, while excessive stress, significant weight loss, or over-exercise can all suppress GnRH and slow the entire downstream cascade.
Eating disorders and very low body fat interrupt that same upstream signal and can cause periods to stop entirely.
What Tracking Looks Like in Practice
A simple cycle tracker that records period start and end dates, flow heaviness, and notable symptoms gives a clinician the kind of timeline that helps pinpoint where the cascade broke down. Patterns across three cycles are more useful than a single snapshot, and whether you choose an app or a paper calendar, the goal is to make your hormonal story easy to read at a glance.
The Bottom Line
Your period starts when progesterone withdraws, with falling estrogen lending a hand, after the corpus luteum breaks down at the end of an unfertilized cycle. That withdrawal is the final step in a relay that began with GnRH from the hypothalamus, moved through FSH and LH from the pituitary, and built up estrogen and progesterone in the ovaries. Understanding the cascade in order makes cycle symptoms easier to place and unusual patterns easier to spot.
FAQ
Which hormone causes your period to start?
The direct trigger is the withdrawal of progesterone after the corpus luteum breaks down at the end of an unfertilized cycle. Falling estrogen contributes to the breakdown, but progesterone withdrawal is the central event that destabilizes and sheds the endometrium.
What is the main hormone that triggers your period to start?
A sharp drop in progesterone is the primary hormonal event that triggers the start of your period. Once the corpus luteum dissolves around day 22 to 24 of a typical cycle, both progesterone and estrogen fall sharply, and the lining loses the structural support it needs to stay intact.
How does estrogen affect the start of menstruation?
Estrogen thickens the endometrium during the first half of the cycle, then falls late in the luteal phase. That late drop removes one of the signals keeping the lining healthy, so when estrogen and progesterone both fall together, the lining breaks down and bleeds.
What role does progesterone play in triggering a period?
Progesterone stabilizes the thickened uterine lining during the luteal phase, keeping blood vessels firm and tissue structure intact. When progesterone drops sharply after the corpus luteum dissolves, that structural support collapses and the lining sheds.
How do LH and FSH hormones relate to the start of a period?
LH and FSH start the chain of events that eventually leads to a period. FSH recruits ovarian follicles and drives estrogen production, and the mid-cycle LH surge triggers ovulation, which sets up the corpus luteum whose later breakdown causes the progesterone withdrawal that starts bleeding.
What hormonal drop causes the uterine lining to shed?
The sharp drop in progesterone is the primary event, with falling estrogen playing a supporting role. This combined withdrawal removes both the structural stability and the growth signal the endometrium depends on, causing the lining to break down and shed.
