Your period starts when progesterone levels drop sharply at the end of your menstrual cycle. This hormone drop is the single most important trigger, setting off a chain reaction that breaks down the uterine lining and pushes it out as menstrual flow. The whole process is orchestrated by a feedback loop between your brain and your ovaries that runs on a roughly 28-day clock, though cycles anywhere from 21 to 35 days are normal.
The Progesterone Drop That Starts Everything
For the two weeks after ovulation, a temporary structure on your ovary called the corpus luteum pumps out progesterone. This hormone’s job is to maintain the thick, blood-rich lining your uterus has built up in case a fertilized egg needs to implant. Think of progesterone as the signal that keeps the lining intact and stable.
If no pregnancy occurs within about 10 to 12 days after ovulation, the corpus luteum breaks down. Progesterone production drops rapidly, and that withdrawal is what your body reads as the “go” signal for menstruation. Without progesterone holding things together, several things happen almost simultaneously in the uterine lining: inflammation ramps up, tissue-dissolving enzymes activate, and blood vessels constrict. Each of these plays a direct role in shedding the lining.
How the Lining Actually Breaks Down
Progesterone doesn’t just passively hold the lining in place. It actively suppresses inflammation and keeps certain destructive enzymes locked away. When progesterone falls, those brakes come off all at once.
First, a wave of inflammatory signaling molecules activates inside the cells of the uterine lining. These trigger the production of tissue-dissolving enzymes that start breaking apart the structural framework holding the lining together, specifically in the upper layers of the endometrium. At the same time, progesterone had been keeping prostaglandin levels low by boosting an enzyme that breaks prostaglandins down. Once progesterone drops, prostaglandin levels surge.
Prostaglandins do two things that directly cause your period. They constrict the tiny spiral-shaped arteries that supply blood to the uterine lining, cutting off oxygen and causing the tissue to die. They also trigger the muscular wall of the uterus to contract, which helps expel the dead tissue. Those contractions are what you feel as cramps. The combination of oxygen deprivation, tissue breakdown, and muscular squeezing is what produces menstrual flow.
Your Brain Sets the Timeline
The timing of all this is controlled from much higher up. A small region of your brain releases a signaling hormone in pulses throughout the cycle, and the speed of those pulses changes everything. During the first half of the cycle, pulses come every 60 to 90 minutes, which promotes the hormones that mature an egg and trigger ovulation. After ovulation, progesterone slows the pulse rate down to about one every 200 minutes, shifting the hormonal balance toward maintaining the uterine lining.
At the end of the cycle, when progesterone and estrogen both fall, the pulse rate speeds up again. This reset is what kicks off a new cycle: faster pulses stimulate the hormones that will begin maturing a fresh egg, and the whole process starts over.
Why the Luteal Phase Controls Your Timing
If your cycle length varies from month to month, the variation almost always comes from the first half of the cycle, before ovulation. The second half, called the luteal phase, is remarkably consistent. In a large population study, the average luteal phase lasted 11.8 days, with most people falling between 10.5 and 13 days. That consistency held across different age groups.
This means the corpus luteum has a fairly fixed lifespan. Once you ovulate, you can generally expect your period about 12 to 14 days later, regardless of how long the first half of your cycle took. A 35-day cycle and a 25-day cycle typically have the same luteal phase length. The difference is how long it took to ovulate.
What Can Delay or Shift Your Period
Since the entire trigger depends on progesterone dropping, anything that affects progesterone levels or ovulation timing will shift when your period starts. Stress, significant weight changes, intense exercise, and illness can all delay ovulation, which pushes your whole cycle later. You haven’t “skipped” a period in most of these cases. Your body simply took longer to ovulate, so the countdown to menstruation started later.
Hormonal birth control works by overriding this natural system entirely. Methods that supply synthetic progesterone keep the lining thin and stable. The bleeding you get during a placebo week isn’t a true period triggered by the corpus luteum breaking down. It’s a withdrawal bleed caused by stopping the external hormones.
Even over-the-counter anti-inflammatory medications like ibuprofen can have a small effect, because they reduce prostaglandin production. At high doses (around 800 milligrams every six hours), ibuprofen can delay a period by up to a day or two and reduce flow by roughly 10% to 20%. But this isn’t reliable enough to meaningfully control your cycle. It simply blunts one part of the shedding process without changing the underlying hormonal trigger.
Putting the Sequence Together
The full chain of events looks like this: your brain’s pulse generator drives ovulation, which creates the corpus luteum, which produces progesterone, which maintains the uterine lining. When no pregnancy occurs, the corpus luteum degrades, progesterone plummets, inflammatory signals activate, tissue-dissolving enzymes break down the lining, prostaglandins constrict blood vessels and trigger contractions, and the lining sheds. Day one of bleeding is day one of your next cycle, and the brain’s signaling pulses are already speeding up to start the process again.
Every step depends on the one before it. A period isn’t something that happens on a simple timer. It’s the end result of a hormonal conversation between your brain, your ovaries, and your uterus, with progesterone withdrawal as the final, decisive signal.