Several hormones are active during your period, but the one most directly responsible for menstrual bleeding is prostaglandin, a hormone-like substance released by the uterine lining that triggers the contractions needed to shed it. At the same time, estrogen and progesterone are at their lowest points of the entire cycle, and follicle-stimulating hormone (FSH) is quietly rising to kick off the next round of egg development. Understanding what each of these does helps explain not just why you bleed, but why you feel the way you do during those days.
The Hormone Drop That Starts It All
Your period doesn’t begin because a hormone spikes. It begins because two hormones crash. When a released egg goes unfertilized, the structure that produced it (called the corpus luteum) breaks down after about 14 days. That breakdown causes estrogen and progesterone to plummet. Since progesterone was the hormone keeping the uterine lining thick and stable, its withdrawal is the direct trigger for shedding. Think of it like pulling the support beams out from under a structure: the lining loses its blood supply and begins to break apart.
During the first few days of bleeding, estrogen levels sit at roughly 50 to 100 pg/mL, which is the lowest they’ll be all month. Progesterone is similarly bottomed out. This hormonal low point is also why many people feel tired, flat, or emotionally drained at the start of their period. The mood-stabilizing effects of progesterone and the energy-boosting effects of estrogen are both at their weakest.
Prostaglandins: The Real Engine of Your Period
Once progesterone drops, the cells of the uterine lining release prostaglandins. These aren’t classical hormones made by glands. They’re chemical messengers produced locally in the tissue, and they do two critical jobs: they cause the muscular wall of the uterus to contract, and they constrict the blood vessels feeding the lining. Together, those actions squeeze out and expel the tissue your body no longer needs.
Prostaglandin levels are highest on the first day of your period, which is why day one is usually the heaviest and most uncomfortable. The specific type most involved is called PGF2α. It forces intense, rhythmic contractions of the uterine muscle while simultaneously cutting off oxygen to the lining, causing the tissue to die and detach. A second type, PGE2, adds to the effect. Research measuring prostaglandin levels in blood drawn on day one of menstruation found a direct, statistically significant correlation between higher PGF2α levels and more severe cramps. In plain terms, the more prostaglandin your body produces, the worse your cramps tend to be.
This is exactly why ibuprofen and similar anti-inflammatory painkillers work so well for period pain. They block the enzyme that makes prostaglandins. Taking them early, ideally at the first sign of bleeding or cramping, reduces the total amount of prostaglandin your uterus produces and can meaningfully lower both pain and flow.
FSH: The Quiet Hormone Already Building Your Next Cycle
Even while you’re actively bleeding, your body is already preparing for the next cycle. With estrogen and progesterone out of the way, a region of the brain called the pituitary gland starts releasing follicle-stimulating hormone (FSH). FSH does exactly what its name suggests: it stimulates small follicles in the ovaries to begin growing, each one containing an immature egg.
This process begins within the first few days of your period. Fertility testing measures FSH on day three of the cycle for this reason; it’s the moment when FSH is actively recruiting follicles and its level reflects how hard the body is working to get ovulation going. Normal FSH values for adults after puberty range from about 4.7 to 21.5 mIU/mL. Over the next week or so, one follicle will become dominant, and the rising estrogen it produces will eventually stop the bleeding and start thickening a fresh uterine lining.
How Hormonal Birth Control Changes the Picture
If you’re on hormonal birth control, the bleeding you experience during your placebo week isn’t technically a period. It’s a withdrawal bleed caused by stopping the synthetic hormones in your pills, patch, or ring. The underlying hormonal shift is similar in concept (hormones drop, lining sheds), but different in degree. Because the synthetic hormones in birth control prevent the uterine lining from thickening as much as it normally would, there’s simply less tissue to shed. The hormone shift is also less dramatic, which is why PMS symptoms, cramp severity, and bleeding volume are often noticeably milder on birth control compared to a natural cycle.
Other Hormones at Play
A few other hormonal players are worth knowing about. Luteinizing hormone (LH) is low during your period, similar to estrogen and progesterone, but it works in tandem with FSH to eventually trigger ovulation later in the cycle. The brain’s signaling hormone, gonadotropin-releasing hormone (GnRH), pulses from the hypothalamus roughly every 60 to 120 minutes and controls how much FSH and LH the pituitary releases. Changes in this pulse rate throughout the cycle are what orchestrate the entire hormonal sequence.
Relaxin, a hormone better known for its role in pregnancy, also fluctuates. Its levels drop if conception doesn’t occur, falling as the new cycle begins. Some research suggests relaxin may contribute to heavier menstrual bleeding, though this isn’t fully confirmed.
Why This Matters for How You Feel
The combination of rock-bottom estrogen, minimal progesterone, and a surge of prostaglandins creates a unique physiological state. Low estrogen can contribute to headaches, fatigue, and reduced pain tolerance. Low progesterone removes its calming effect on the nervous system. And prostaglandins don’t just act on the uterus. They can spill into the bloodstream and affect the digestive tract, which is why loose stools, nausea, and even diarrhea are common during the first day or two of a period.
The good news is that this hormonal low point is temporary. As FSH does its work and a new follicle begins producing estrogen, typically by day four or five, energy and mood often start to climb. By the time bleeding stops, estrogen is on a steady upward trajectory, and many people notice a real shift in how they feel heading into the first week after their period ends.