During your period, the uterus actively sheds the inner lining it spent the previous weeks building up. This involves a coordinated sequence of hormonal shifts, blood vessel changes, tissue breakdown, and muscle contractions that typically lasts between 3 and 5 days. Understanding what’s happening inside can help make sense of the cramping, bleeding, and other sensations that come with menstruation.
The Hormonal Signal That Starts It All
The entire process begins with a drop in progesterone. Throughout the second half of your cycle, progesterone keeps the uterine lining thick, stable, and blood-rich in case a fertilized egg implants. When pregnancy doesn’t occur, the ovary stops producing progesterone, and levels fall sharply. This decline triggers a cascade of events inside the uterine lining that, after a critical window, become irreversible. Once that threshold is crossed, tissue breakdown and bleeding are inevitable.
The progesterone drop activates enzymes that begin dissolving the structural framework holding the lining together. These enzymes break down the proteins and connective tissue that give the lining its shape, loosening it from the uterine wall so it can be expelled.
Which Layer Sheds and Which Stays
The uterine lining (endometrium) has two distinct layers. The deeper layer sits against the muscle wall and is permanent. It never sheds. The upper layer is the one that thickens each cycle in preparation for pregnancy, and it’s the one that breaks down completely during your period. By the end of menstruation, only the thin base layer remains, measuring roughly 5.4 millimeters on average around day 7 or 8 of the cycle. That base layer then serves as the foundation for regrowing a fresh lining in the next cycle.
How the Blood Supply Shuts Off
The upper lining is fed by small, coiled blood vessels called spiral arteries. These are the terminal branches of the arteries that supply the uterus, and they’re highly sensitive to hormonal signals. As progesterone drops, these arteries constrict, cutting off blood flow to the upper lining. This creates a period of oxygen deprivation that causes the tissue to die. The arteries then relax, and blood flows into the now-damaged tissue, flushing out the loosened lining as menstrual fluid.
Later, a muscular segment near the base of these vessels constricts again to slow and eventually stop the bleeding. This natural shutoff mechanism is what keeps menstrual blood loss within a normal range for most people.
Uterine Contractions and Cramping
Your uterus is surrounded by a thick layer of muscle, and during your period, that muscle contracts rhythmically to help push out the shedding lining. These contractions are driven by prostaglandins, chemical signals released by the dying tissue. Prostaglandins cause muscle cells in the uterine wall to flood with calcium, which triggers them to contract forcefully.
This is what you feel as menstrual cramps. The contractions themselves squeeze the small blood vessels in the uterine wall, temporarily cutting off oxygen to the muscle. That combination of strong contractions and reduced blood flow creates the aching, pressure-like pain that’s characteristic of period cramps. People with higher prostaglandin levels tend to experience more intense contractions and more pain. It’s the same basic mechanism behind labor contractions, just at a much smaller scale.
What Menstrual Fluid Actually Contains
What leaves your body during a period isn’t purely blood. Menstrual fluid is a mix of blood, shed tissue from the uterine lining, vaginal secretions, and mucus. Proteomic analysis has identified over 385 unique proteins in menstrual fluid that aren’t found in regular blood or vaginal fluid alone, reflecting the complex tissue breakdown happening inside the uterus. The small clots you sometimes see are pieces of the lining that came away in larger fragments.
The total volume is less than most people assume. A study using direct measurement with vaginal cups found the average menstrual volume is about 87 milliliters across an entire period, with a typical range of 15 to 271 milliliters. Half of all participants fell at or below 81 milliliters. For people who have given birth, volumes tend to run slightly higher, with anything over 169 milliliters considered outside the normal range.
Changes to the Cervix
The cervix, the narrow opening at the base of the uterus, adjusts its position and shape to allow menstrual fluid to pass through. During your period, the cervix sits lower in the body than it does at other points in the cycle, making it easier to reach if you were to check it. It also opens slightly, enough to let blood and tissue flow out into the vaginal canal. After menstruation ends, the cervix rises higher, firms up, and closes again.
How the Uterus Rebuilds
Even before your period fully ends, the repair process is already underway. The base layer of the endometrium begins generating new tissue, stimulated by rising estrogen levels as the next cycle’s follicle develops in the ovary. New blood vessels grow, new glands form, and the lining thickens steadily over the following 7 to 10 days. By the time ovulation occurs around the middle of the cycle, the lining has rebuilt to a thick, nutrient-rich surface ready to support a potential pregnancy. If that doesn’t happen, the whole process starts again.
This cycle of building, maintaining, and shedding the lining repeats roughly every 28 days, though anywhere from 21 to 35 days is typical. The uterus is one of the few organs in the body that undergoes this kind of regular, large-scale tissue turnover throughout reproductive life.