Your period happens when hormone levels drop at the end of your monthly cycle, causing the lining of your uterus to shed. This process is part of a roughly 28-day hormonal cycle that repeats from puberty until menopause, typically starting between ages 12 and 13. Understanding what drives it can help you recognize what’s normal, what to expect, and what your body is actually doing each month.
What Triggers a Period
The short answer: a drop in progesterone. Throughout the second half of your cycle, your ovaries produce progesterone to maintain the thickened lining of the uterus. If a fertilized egg doesn’t implant, progesterone levels fall sharply. That withdrawal is the direct trigger for menstruation. The blood vessels supplying the uterine lining constrict, the tissue breaks down, and your body sheds it through the vagina along with blood and mucus.
A normal period lasts 2 to 7 days. Most people lose less than 80 milliliters of blood total, which is roughly 5 to 6 tablespoons spread across the entire period. It often looks like more because menstrual fluid also contains tissue and other fluids.
How Your Brain Controls the Cycle
Your menstrual cycle isn’t just an ovary event. It starts in your brain. A small region called the hypothalamus releases a signaling hormone in pulses, which tells your pituitary gland (a pea-sized gland at the base of the brain) to release two key hormones into your bloodstream. One stimulates your ovaries to develop egg-containing follicles. The other triggers the release of a mature egg at ovulation.
These hormones work in a feedback loop. As your ovarian follicles grow, they produce rising levels of estrogen. That estrogen signals back to the pituitary, gradually increasing its sensitivity. Just before ovulation, estrogen levels peak, triggering a sudden surge of hormones from the pituitary that causes the follicle to release the egg. After ovulation, the empty follicle transforms into a structure that pumps out progesterone, and the cycle continues until progesterone drops and your period begins.
The Four Phases of Your Cycle
A typical cycle runs 21 to 35 days, counted from the first day of one period to the first day of the next. It breaks into four overlapping phases.
Menstruation is the phase you can see. It typically lasts 2 to 7 days as your body sheds the uterine lining. Day one of bleeding is day one of your cycle.
The follicular phase overlaps with menstruation and extends beyond it, lasting about 13 to 14 days total. During this time, rising hormone levels stimulate several follicles in your ovaries to develop. Usually one becomes dominant, producing increasing amounts of estrogen that thicken the uterine lining in preparation for a potential pregnancy.
Ovulation happens around the midpoint of your cycle when the surge from your pituitary gland releases the mature egg from its follicle. The egg travels into the fallopian tube, where it can be fertilized for roughly 12 to 24 hours.
The luteal phase fills the second half of your cycle, generally lasting about 14 days. The empty follicle produces progesterone, which stabilizes the uterine lining and prepares it for implantation. If pregnancy doesn’t occur, progesterone drops, and menstruation starts again.
When Periods Typically Start
Most people get their first period between ages 12 and 13, though anywhere from 9 to 15 is within the normal range. By age 15, about 98% of adolescents will have started menstruating. The first period usually arrives about two to three years after breast development begins.
In the first year or two, cycles are often irregular. It’s common to skip months, have longer or shorter cycles, or experience unpredictable flow. This happens because the hormonal feedback loop between the brain and ovaries is still maturing. Cycles typically become more regular within two to three years of the first period.
Signs Your Period Is Coming
The hormonal shifts in the days before your period can produce a range of physical and emotional changes, commonly called PMS. These symptoms are driven by the drop in progesterone and estrogen, along with fluctuations in serotonin, a brain chemical involved in mood regulation.
Common physical signs include:
- Breast tenderness
- Abdominal bloating and fluid retention
- Fatigue
- Headaches
- Acne flare-ups
- Muscle or joint pain
- Constipation or diarrhea
Mood changes, food cravings, irritability, and trouble sleeping are also common. Lower serotonin levels in the premenstrual window may contribute to these emotional symptoms. PMS symptoms typically start a few days to a week before bleeding begins and resolve once your period is underway.
What Counts as a Normal Cycle
A healthy cycle falls between 21 and 35 days, with bleeding lasting 2 to 7 days. There’s a lot of individual variation, and your cycle length can shift due to stress, weight changes, exercise, illness, or travel. A few days of variation from month to month is perfectly normal.
Cycles that consistently fall outside the 21-to-35-day window are worth paying attention to. If you’ve been having regular periods and then miss three in a row (without pregnancy), that meets the clinical definition of secondary amenorrhea and is worth investigating. The same applies if your periods have always been irregular and you go six months without one. On the other end, periods arriving more frequently than every 21 days or lasting longer than a week can signal hormonal imbalances, thyroid issues, or structural changes in the uterus.
Tracking your cycle for a few months, even with a simple calendar, gives you a baseline. Knowing your own pattern makes it much easier to spot when something shifts.