Menstruation is the monthly shedding of the uterine lining through the vagina. It happens when a released egg isn’t fertilized, signaling the body to clear the lining it built up in preparation for pregnancy and start the cycle over. A typical period lasts 2 to 7 days, and the full menstrual cycle (from the first day of one period to the first day of the next) ranges from 21 to 35 days.
What Menstrual Fluid Actually Contains
Menstrual fluid isn’t pure blood. It’s a mixture of discarded uterine lining cells, blood, vaginal fluid, and sometimes stretchy cervical mucus. This is why it often looks different from blood you’d see from a cut: it can be darker, thicker, or contain small clots. The color can range from bright red to dark brown depending on how quickly it leaves the body.
How Hormones Drive the Cycle
The menstrual cycle is controlled by a feedback loop between three parts of the body: the hypothalamus (a small region in the brain), the pituitary gland (just below it), and the ovaries. The hypothalamus sends out a signal hormone in pulses, which tells the pituitary to release two key hormones that act on the ovaries. One of these stimulates egg-containing follicles to grow and produce estrogen. The other triggers ovulation, the release of a mature egg, roughly mid-cycle.
After ovulation, the empty follicle transforms into a temporary structure that pumps out progesterone. Progesterone is the hormone responsible for thickening and maintaining the uterine lining. If no pregnancy occurs, this structure breaks down, and both progesterone and estrogen levels drop sharply. That drop in progesterone is the direct trigger for menstruation. The upper layer of the uterine lining, called the functional layer, breaks down and sheds, then rebuilds itself over the following weeks as hormone levels rise again.
The Four Phases of the Cycle
The menstrual phase is the period itself: the days of active bleeding when the lining is shed. This is when hormone levels are at their lowest.
The follicular phase overlaps with and then follows menstruation. During this phase, the pituitary gland ramps up its signal to the ovaries, and several follicles begin developing. Rising estrogen from these growing follicles prompts the uterine lining to start thickening again. This phase is the most variable in length, which is why cycles differ from person to person and month to month.
Ovulation happens when estrogen levels climb high enough to flip from suppressing the pituitary’s output to amplifying it. This triggers a surge that causes one dominant follicle to release its egg. This positive feedback mechanism is unique to the female reproductive system and is what makes the cycle, well, cyclical. Ovulation typically occurs around day 14 of a 28-day cycle, but it can shift significantly.
The luteal phase follows ovulation and lasts roughly 12 to 14 days. Progesterone peaks during this phase, preparing the uterine lining for a potential embryo. If no implantation occurs, the cycle resets with menstruation.
When Periods Typically Start and Stop
The first period, called menarche, occurs at a median age of about 11.9 years in the United States, with the average sitting around 12.5. It’s common for cycles to be irregular for the first couple of years as the hormonal feedback system matures. On the other end, most people reach menopause (the permanent end of menstruation) in their late 40s to early 50s.
Why Cramps Happen
Menstrual cramps are caused by substances called prostaglandins that the uterine lining produces as it breaks down. Prostaglandins trigger the muscle of the uterus to contract, which helps expel the lining. Higher levels of prostaglandins mean stronger contractions and more pain. This is why anti-inflammatory pain relievers, which reduce prostaglandin production, tend to work well for period cramps. The pain is usually strongest in the first day or two of bleeding, when prostaglandin levels are highest.
Some people also experience bloating, headaches, fatigue, or loose stools during their period. These aren’t random: prostaglandins can affect nearby tissues, including the bowel, which is why digestive changes during menstruation are so common.
What Counts as a Normal Period
A normal cycle falls between 21 and 35 days, and bleeding lasts 2 to 7 days. Cycle length can vary by a few days from month to month without being a concern. The amount of blood lost during a period is often less than people expect. The medical threshold historically used for “heavy” bleeding is about 80 milliliters (roughly 5 to 6 tablespoons) over the entire period, though that number is hard to measure in practice.
More practical signs that bleeding is unusually heavy or that something may need attention include:
- Soaking through a pad or tampon every hour for more than two hours in a row
- Bleeding that lasts longer than 7 days
- Cycles shorter than 21 days or longer than 35 days
- Cycle lengths that swing by more than 7 to 9 days from one month to the next
- Spotting between periods or after sex
- Missing your period for 3 to 6 months (outside of pregnancy)
If you’re soaking through pads or tampons hourly and also feeling dizzy, lightheaded, or short of breath, that combination warrants emergency care because it can signal significant blood loss.
Menstrual Product Safety
Tampons, pads, menstrual cups, and discs are all widely used. The main safety consideration with internal products is leaving them in too long, which can increase the risk of toxic shock syndrome, a rare but serious bacterial infection. Practical guidelines for reducing that risk: change tampons every four to eight hours, use the lowest absorbency you need, and don’t wear tampons when you’re not on your period. Menstrual cups and cervical caps carry a similar principle: anything that stays in the vagina for extended periods raises risk, so following the manufacturer’s recommended wear time matters.