Periods come from the shedding of the inner lining of the uterus, called the endometrium, when pregnancy doesn’t occur after ovulation. Each month, your body builds up this lining with extra blood vessels and tissue to prepare for a fertilized egg. When no egg implants, hormone levels drop sharply, and the lining breaks down and exits through the vagina as menstrual fluid.
The whole process is driven by a cycle of rising and falling hormones that repeat roughly every 28 days, though anywhere from 21 to 35 days is normal.
The Hormonal Chain Reaction
Your period is the end result of a conversation between three parts of your body: a small region at the base of the brain (the hypothalamus), a pea-sized gland just below it (the pituitary), and your ovaries. The hypothalamus kicks things off by releasing a signaling hormone that tells the pituitary to produce two key hormones. The first stimulates a group of egg-containing follicles in the ovary to start maturing. As these follicles grow, they pump out estrogen, which thickens the uterine lining with new blood vessels, glands, and tissue. By the end of this building phase, the lining reaches 8 to 12 millimeters thick.
Around the midpoint of the cycle, a surge of the second pituitary hormone triggers ovulation, the release of a mature egg from one of the ovaries. The empty follicle left behind transforms into a temporary structure that starts producing progesterone. Progesterone is the hormone that makes the lining “sticky” and nutrient-rich, creating the ideal environment for a fertilized egg to implant. It stimulates the growth of complex glands within the lining and increases glycogen (stored energy) in the tissue.
If fertilization doesn’t happen within about two weeks, that temporary structure in the ovary breaks down and stops producing hormones. This is the moment that sets a period in motion.
What Triggers the Actual Bleeding
The sharp drop in both progesterone and estrogen at the end of the cycle is the direct trigger for your period. Without these hormones, the thickened uterine lining can no longer sustain itself. The blood vessels that supply the upper portion of the lining, called spiral arteries, begin to coil tightly and constrict. This cuts off blood flow to the surface layers, starving the tissue of oxygen.
As the tissue dies, it loosens and begins to separate from the uterine wall. At the same time, your body releases prostaglandins, chemical messengers that cause the muscular wall of the uterus to contract. These contractions help push the detaching tissue out. Higher levels of prostaglandins mean stronger contractions, which is why some people experience more intense cramps than others.
Only the upper two-thirds of the endometrium sheds. The deeper base layer stays intact. This is a carefully controlled process: the inflammation and breakdown are confined to the surface, sparing the base layer so it can immediately begin regenerating a new lining once the period ends.
What Menstrual Fluid Actually Contains
What comes out during a period isn’t purely blood. Menstrual fluid is a mix of blood, endometrial cells shed from the uterine wall, immune cells, 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, all of which are normal.
The total amount lost during a single period is typically around 60 milliliters, or roughly 4 tablespoons. That’s far less than most people assume. Anything above 80 milliliters per cycle is considered heavy bleeding and may warrant a closer look, especially if it’s accompanied by fatigue or dizziness.
The Four Phases of the Cycle
The menstrual cycle has two overlapping timelines: one tracking what happens in the ovaries and another tracking changes in the uterine lining. Together, they break down into four main phases.
- Menstrual phase (days 1 to 5, approximately): The lining sheds. Estrogen and progesterone are at their lowest. The pituitary gland, sensing these low hormone levels, starts releasing signals to begin growing new follicles in the ovary even before bleeding finishes.
- Follicular/proliferative phase (days 1 to 14): Overlapping with menstruation, follicles in the ovary begin maturing and producing estrogen. Rising estrogen rebuilds the uterine lining from its base layer, thickening it with new tissue and blood supply.
- Ovulation (around day 14): A hormone surge causes the dominant follicle to rupture and release an egg. This typically happens 36 to 44 hours after the surge begins. The egg travels into the fallopian tube.
- Luteal/secretory phase (days 15 to 28): The leftover follicle produces progesterone, which transforms the lining into a receptive surface for implantation. This phase lasts about 14 days. If no pregnancy occurs, the structure degrades, hormones plummet, and menstruation begins again.
Why Cramps Happen
Period cramps are a direct result of the prostaglandins your uterus releases to help expel its lining. These compounds trigger muscle contractions in the uterine wall and also play a role in inflammation and pain signaling. The contractions briefly squeeze the blood vessels in the uterus, temporarily reducing oxygen supply to the muscle, which intensifies the aching or cramping sensation. People who produce more prostaglandins tend to have more severe pain, and this variation is largely biological rather than a matter of pain tolerance.
When Periods Typically Start
Most people get their first period between ages 10 and 15, with the average falling around age 12. The medical term for this is menarche. It usually happens about two years after breast development begins and is one of the later milestones of puberty, not the first. Cycles in the first year or two are often irregular because the hormonal feedback loop between the brain and ovaries is still calibrating. It can take several years before cycles settle into a predictable rhythm.
Cycle length varies widely from person to person and can also shift throughout a person’s reproductive years due to stress, weight changes, exercise, and aging. Periods typically continue until menopause, which occurs on average around age 51, when the ovaries stop releasing eggs and hormone production declines permanently.