A period happens when dropping levels of the hormone progesterone trigger your uterus to shed its inner lining. This process is the final step in a roughly 28-day cycle of hormonal signals between your brain, ovaries, and uterus. About 91% of women have cycles that fall between 21 and 35 days, and a typical period releases around 60 milliliters (about 2.7 ounces) of fluid.
The Hormonal Chain Reaction
Your menstrual cycle is controlled by a communication loop between three parts of your body: a small region at the base of your brain called the hypothalamus, the pituitary gland just below it, and your ovaries. The hypothalamus kicks things off by releasing a signaling hormone that tells the pituitary gland to produce two key hormones. The first stimulates follicles (tiny fluid-filled sacs containing eggs) in your ovaries to grow. As a follicle matures, it releases estrogen, which thickens the lining of your uterus in preparation for a possible pregnancy.
When estrogen rises high enough, the pituitary gland releases a surge of a second hormone that triggers ovulation, the release of an egg from the ovary. The empty follicle left behind transforms into a temporary structure called the corpus luteum, which starts pumping out progesterone. Progesterone is the hormone that maintains and enriches the uterine lining, keeping it spongy and blood-vessel-rich so a fertilized egg could implant.
If the egg isn’t fertilized, the corpus luteum begins to break down around 10 days after ovulation. As it degrades, progesterone and estrogen levels plummet. That drop in progesterone is the direct trigger for your period. Without progesterone supporting it, the uterine lining can no longer sustain itself. Research in animal models has shown that there’s a critical window of about 12 to 16 hours: if progesterone is restored within the first 12 hours, shedding can be blocked entirely, but after 16 hours, the process becomes irreversible.
What Happens Inside Your Uterus
Your uterine lining, the endometrium, has two distinct layers. The deeper layer sits against the muscular wall of the uterus and stays put throughout your cycle. It acts like a root system, containing a network of gland structures that creep along the muscle layer. The upper layer is the one that grows fresh each month and sheds during your period.
When progesterone drops, the blood vessels feeding that upper layer respond dramatically. These are tightly coiled vessels called spiral arteries, and they begin to constrict. Classic physiological studies found that in the days before bleeding starts, the tissue shrinks by 25 to 75%, the spiral arteries become even more tightly coiled, and blood flow slows to a near stop, with red blood cells sometimes pausing for 60 to 90 seconds at a time. This cuts off oxygen to the upper layer of the lining, essentially starving it.
After a period of intense constriction, the blood vessels suddenly dilate and reopen. That’s when bleeding begins. The tissue, now damaged from oxygen deprivation, breaks apart and exits the body along with blood from the reopened vessels.
Why Periods Hurt
The cramping that often accompanies a period comes from chemical messengers called prostaglandins. As the lining breaks down, cells in the uterus release prostaglandins, which cause the muscular wall of the uterus to contract. These contractions physically squeeze the shedding tissue out of the uterus and through the cervix.
Prostaglandins are necessary for this process, but the amount your body produces matters. Higher levels of prostaglandins cause stronger, more frequent contractions, which translates to more intense cramping. Excess prostaglandins are also linked to heavier bleeding. This is why over-the-counter pain relievers that block prostaglandin production tend to be effective for period cramps.
What Menstrual Fluid Actually Contains
What comes out during your period isn’t purely blood. Menstrual fluid is a mix of blood, shed endometrial cells from the uterine lining, and vaginal secretions. This is why it often looks different from blood you’d see from a cut: it can be darker, thicker, or contain small clots and tissue fragments. The total volume averages about 60 milliliters over the course of a period. Losing more than 80 milliliters regularly is considered heavy menstrual bleeding.
How the Cycle Resets
The same hormone drop that triggers your period also resets the entire system. When progesterone and estrogen fall low enough, they stop suppressing the brain’s signaling hormones. This lifts the brakes on the pituitary gland, which begins releasing follicle-stimulating hormone again. New follicles start growing in the ovaries, estrogen rises, and the uterine lining begins rebuilding from its deeper root layer, even while you’re still bleeding. By the time your period ends, the next cycle is already well underway.
This feedback loop is why cycle length can vary. Anything that delays ovulation, such as stress, significant weight changes, or hormonal conditions, pushes the entire timeline back. A longer first half of the cycle means a later period. The second half, from ovulation to the start of bleeding, is more consistent at roughly 14 days, because the corpus luteum has a fairly fixed lifespan.