A period starts when progesterone and estrogen levels drop sharply at the end of your menstrual cycle. This hormone withdrawal is the direct trigger that causes your uterine lining to break down and shed. But the process involves a chain of hormonal events, not just a single switch, and understanding that chain helps explain why periods sometimes arrive late, early, or not at all.
Progesterone Withdrawal: The Primary Trigger
The single most important hormonal event behind your period is the fall in progesterone. During the second half of your cycle (the luteal phase), a temporary structure on your ovary called the corpus luteum pumps out progesterone. This hormone’s job is to maintain the thickened uterine lining in case a fertilized egg implants. If pregnancy doesn’t happen, the corpus luteum breaks down in a process called luteolysis. In humans, progesterone levels plummet over roughly 48 hours as this structure degrades.
That rapid drop is what sets menstruation in motion. Progesterone had been keeping the lining stable, nourished, and blood-rich. Without it, the lining loses its hormonal support and begins to degenerate. Estrogen drops alongside progesterone during this window, and the combined withdrawal of both hormones, particularly progesterone, is what initiates bleeding.
What Happens Inside the Uterus
Once progesterone falls, the uterine lining goes through a specific sequence of physical changes. The first visible step is constriction of the spiral arteries, tiny coiled blood vessels that supply the upper layer of the lining. Research in Physiological Reviews describes transient, intense vasoconstriction of these arteries occurring 4 to 24 hours before menstruation begins, directly following progesterone withdrawal.
When these arteries clamp down, they cut off blood flow to the upper lining. That creates a low-oxygen environment, essentially starving the tissue. The oxygen-deprived lining starts to break apart, and the damaged tissue triggers an inflammatory response. This inflammation is a normal, necessary part of the process. It helps detach the upper layer of the lining while leaving the deeper base layer intact so the lining can rebuild during the next cycle.
Eventually the constricted arteries relax, and blood flows back into the damaged tissue. This is what produces menstrual bleeding: blood released from the now-open vessels mixes with the broken-down lining tissue and exits through the cervix.
The Role of Prostaglandins
Prostaglandins are chemical messengers produced locally in the uterine lining, and they play a critical role in making your period happen. One type in particular, prostaglandin F2-alpha, drives both the blood vessel constriction and the uterine contractions you feel as cramps. A 2024 study confirmed that blocking the receptor for this prostaglandin significantly suppressed endometrial breakdown and shedding in animal models.
Your body ramps up prostaglandin production in the lining once progesterone drops. These compounds do double duty: they help squeeze the spiral arteries shut (contributing to the tissue breakdown described above) and they cause the muscular wall of the uterus to contract, pushing out the shedding tissue. Higher prostaglandin levels are directly linked to more painful cramps, which is why anti-inflammatory painkillers that reduce prostaglandin production are effective for period pain.
The Full Hormonal Chain
Your period doesn’t start in isolation. It’s the final step in a cycle-long hormonal conversation between your brain and ovaries. Here’s the sequence in a typical cycle of 24 to 38 days:
- Follicular phase (days 1 through 14 in a 28-day cycle): Your pituitary gland releases follicle-stimulating hormone (FSH), which prompts your ovaries to develop egg-containing follicles. The growing follicles produce rising levels of estrogen, which thickens the uterine lining.
- Ovulation (around day 14): A surge in luteinizing hormone (LH) from the pituitary triggers the release of a mature egg. This is the pivotal event that sets up the rest of the cycle.
- Luteal phase (days 15 through 28): The empty follicle transforms into the corpus luteum and secretes progesterone for roughly 14 days. Progesterone stabilizes and matures the lining. The corpus luteum depends on continued LH support to survive. If LH levels drop and no pregnancy hormone (hCG) arrives to rescue it, the corpus luteum degrades.
- Menstruation: Progesterone and estrogen fall. The lining sheds. Day 1 of bleeding is day 1 of a new cycle.
The luteal phase is remarkably consistent within an individual, typically lasting 14 days. Most variation in cycle length comes from the follicular phase, which is why your period might arrive a few days early or late depending on when you ovulated.
Why Hormone Withdrawal Causes Symptoms
The drop in hormones doesn’t just affect your uterus. It produces a range of physical symptoms throughout your body. Research comparing symptoms during hormone-containing versus hormone-free intervals found that pelvic pain affected 70% of women during the withdrawal window compared to 21% during the rest of the cycle. Bloating or swelling jumped from 19% to 58%, headaches from 53% to 70%, and breast tenderness from 16% to 38%.
These symptoms are a direct consequence of falling progesterone and estrogen levels. Progesterone influences fluid retention, pain sensitivity, and smooth muscle tone throughout the body, not just in the uterus. So when levels plummet, your whole system feels the shift. This is also why premenstrual symptoms often peak in the day or two before bleeding starts, right when hormone levels are dropping fastest.
When Ovulation Doesn’t Happen
True menstruation requires ovulation. If you don’t ovulate during a cycle, no corpus luteum forms, and your body doesn’t produce the progesterone surge that normally sustains and then withdraws support from the lining. You can still experience bleeding in these anovulatory cycles, but the mechanism is different. Instead of progesterone withdrawal causing an organized breakdown, the lining keeps building under estrogen’s influence until it becomes unstable and sheds unevenly. This type of bleeding tends to be unpredictable in timing and flow.
Anovulatory cycles are common during the first few years after periods begin, during perimenopause, and in conditions like polycystic ovary syndrome. The bleeding may look like a period, but hormonally it’s a different process.
How This Changes During Perimenopause
During your peak reproductive years, estrogen rises and falls in consistent patterns, producing regular ovulation and predictable periods. As you enter perimenopause, typically in your 40s, your ovaries begin producing less estrogen. This disrupts the precise hormonal feedback loop that triggers ovulation each month.
With inconsistent ovulation, progesterone production becomes erratic. Some cycles you may ovulate and have a normal progesterone-driven period. Other cycles you may not ovulate at all, leading to longer gaps between periods or unusually light bleeding. Your periods may get shorter, lighter, and less frequent overall, and you might skip months entirely. These changes are among the earliest signs of the transition toward menopause, and they’re all driven by the same core mechanism: shifting levels of the hormones that normally trigger and regulate menstrual bleeding.