Menstruation is caused by the withdrawal of progesterone, the hormone that maintains the uterine lining during the second half of the menstrual cycle. When no pregnancy occurs, a temporary hormone-producing structure in the ovary called the corpus luteum breaks down, progesterone levels drop sharply, and the upper layer of the uterine lining destabilizes and sheds. This process, sometimes called progesterone withdrawal bleeding, is the central event that triggers every menstrual period.
The Corpus Luteum Sets the Clock
After ovulation, the empty follicle that released the egg transforms into the corpus luteum, a small structure that pumps out progesterone for roughly two weeks. Progesterone’s job during this window is to stabilize and enrich the uterine lining so a fertilized egg could implant. The corpus luteum begins to decline 9 to 11 days after ovulation, and without a signal from a pregnancy (specifically, the hormone hCG), it breaks down completely in a process called luteolysis. It eventually becomes inert scar tissue.
That 14-day luteal phase is remarkably consistent from person to person and cycle to cycle, which is why the timing of a period is more predictable than the timing of ovulation. The variable part of your cycle is the first half, when a follicle is maturing. The countdown to menstruation, once ovulation happens, is relatively fixed.
How Progesterone Withdrawal Triggers Bleeding
Progesterone withdrawal is only effective at causing menstruation if the uterine lining has already been built up by estrogen. Estrogen thickens the lining during the first half of the cycle, and progesterone then restructures it into a spongy, blood vessel-rich tissue ready for implantation. When progesterone disappears, that tissue loses its hormonal support and becomes unstable.
The sequence from there is surprisingly mechanical. Falling progesterone ramps up production of chemical messengers called prostaglandins, particularly two types: one that causes the small spiral-shaped arteries in the lining to constrict, and another that triggers the muscular wall of the uterus to contract. The constriction of those spiral arteries cuts off blood flow to the upper layers of the lining, starving the tissue of oxygen. This brief episode of oxygen deprivation causes the tissue to break down and detach. The uterine contractions then help expel it. Those contractions are also why periods come with cramping.
Which Part of the Lining Actually Sheds
The uterine lining has two distinct layers. The upper layer, called the functional layer, is the one that thickens each cycle and sheds during menstruation. The deeper layer, called the basal layer, stays intact. It sits right against the muscular wall of the uterus and contains the root structures of the uterine glands, arranged in a network that persists throughout the cycle regardless of hormonal changes.
After menstruation, the basal layer serves as the regenerative source. Rising estrogen in the next cycle stimulates it to regrow the functional layer from scratch, restarting the process. This is why the lining can rebuild reliably cycle after cycle for decades.
The Brain’s Role in Timing
The hormonal cascade that ultimately leads to menstruation starts much higher up, in a communication loop between the brain and the ovaries. A region of the brain called the hypothalamus releases a signaling hormone (GnRH) in pulses, which tells the pituitary gland at the base of the brain to release two key hormones: FSH, which stimulates follicle growth in the ovary, and LH, which triggers ovulation.
After ovulation, the rising progesterone and estrogen from the corpus luteum feed back to the brain and suppress further release of FSH and LH. This negative feedback loop prevents new follicles from maturing while the body waits to see if pregnancy occurs. When the corpus luteum dies and progesterone drops, that suppression lifts, FSH begins to rise again, and a new follicle starts developing. Menstruation and the start of a new cycle happen simultaneously.
Premenstrual Symptoms and the Hormone Shift
The hormonal changes that precede menstruation don’t just affect the uterus. Falling progesterone and estrogen in the days before your period influence the entire body, which is why premenstrual symptoms can feel so widespread.
Physical symptoms tied to this hormone shift include bloating, breast tenderness, headaches, fatigue, joint or muscle pain, acne flare-ups, and changes in digestion like constipation or diarrhea. Emotional and behavioral changes are equally common: mood swings, irritability, anxiety, difficulty concentrating, food cravings, and trouble sleeping. These symptoms track directly with the hormonal fluctuations, which is why they disappear once the period starts and hormone levels stabilize at their baseline, and why they vanish entirely during pregnancy and menopause, when the cyclical pattern stops.
The prostaglandins responsible for triggering the actual shedding also contribute to symptoms. Higher prostaglandin levels are associated with stronger uterine contractions and more intense cramping. They can also affect nearby organs, which explains why some people experience nausea or diarrhea during the first day or two of their period.
Putting the Full Sequence Together
The entire process follows a clear chain of events. Estrogen builds the uterine lining in the first half of the cycle. Ovulation creates the corpus luteum, which produces progesterone to maintain and mature that lining. Without pregnancy, the corpus luteum degrades 9 to 11 days after ovulation, progesterone plummets, prostaglandin production spikes, the spiral arteries in the lining constrict, oxygen-deprived tissue breaks down, and uterine contractions expel it. The basal layer remains intact and begins regenerating immediately as estrogen rises again in the next cycle.
Every step depends on the one before it. Progesterone withdrawal is the direct trigger, but it only works because estrogen primed the lining, ovulation created the corpus luteum, and the brain’s signaling loop coordinated the entire sequence from the start.