Menstruation is the monthly shedding of the uterine lining, resulting in bleeding from the vagina. It happens when the body prepares for pregnancy each cycle and then, when no pregnancy occurs, breaks down and releases the tissue it built up. A typical period lasts 2 to 7 days, with cycles repeating every 21 to 35 days.
How the Menstrual Cycle Works
A menstrual cycle is counted from the first day of one period to the first day of the next. The whole cycle has two main phases, driven by shifting hormone levels.
In the first phase (the follicular phase), the brain signals the ovaries to start maturing an egg. As the egg develops, the ovaries release rising levels of estrogen. Estrogen causes the uterine lining to thicken with blood-rich tissue, essentially building a cushion that could support a fertilized egg. This phase varies in length, which is why cycles aren’t always the same number of days.
Around the middle of the cycle, a sharp spike in estrogen triggers a surge of another hormone from the brain, which causes the ovary to release a mature egg. This is ovulation. After the egg is released, the structure it left behind on the ovary starts producing progesterone, the dominant hormone of the second phase (the luteal phase). Progesterone stabilizes and maintains the uterine lining, keeping it intact and ready for implantation.
If the egg isn’t fertilized, that structure on the ovary breaks down, and both progesterone and estrogen levels drop sharply. This hormone withdrawal is what directly triggers menstruation. Without progesterone holding things together, the blood vessels in the uterine lining constrict, the tissue loses its blood supply, and it begins to break apart and shed. The lining doesn’t come away all at once. It detaches in a piecemeal fashion, with breakdown and repair happening simultaneously.
What Causes the Bleeding
When progesterone drops, several things happen inside the uterus in quick succession. The lining becomes flooded with inflammatory chemicals, including prostaglandins, which play a big role in both the shedding process and the cramps that come with it. Enzymes that were held in check by progesterone activate and start dissolving the structural framework of the tissue. Immune cells move into the lining and help clear the debris.
The small, coiled blood vessels that fed the lining constrict tightly, cutting off oxygen to the upper layers of tissue. This oxygen deprivation accelerates breakdown. As the tissue releases, those blood vessels open and bleed. The body stops the bleeding through a combination of clotting, vessel constriction, and rapid regrowth of a new surface layer from the base of the uterus. All of this typically resolves within a week.
Normal Blood Loss and Cycle Length
The total amount of blood lost during one period is usually about 60 milliliters, or roughly 2.7 ounces. That’s less than a quarter cup. The normal range spans from 5 to 80 milliliters. Anything consistently above 80 milliliters is considered heavy menstrual bleeding.
Cycles between 24 and 38 days are considered normal. Variation of up to a week from cycle to cycle is also normal. Periods shorter than 2 days or longer than 8 days fall outside typical range.
Common Physical Symptoms
Cramping is the most recognizable symptom, caused by prostaglandins making the uterine muscles contract to help expel the lining. Higher levels of prostaglandins mean stronger contractions and more pain. The cramps are typically felt as a throbbing ache in the lower abdomen, and they tend to be strongest in the first day or two of bleeding.
Prostaglandins don’t stay neatly confined to the uterus. They can affect nearby tissues and the digestive system, which is why many people also experience lower back pain, nausea, diarrhea, and headaches during their period. Over-the-counter anti-inflammatory painkillers work partly by reducing prostaglandin production, which is why they’re effective for period cramps specifically.
Ovulation and the Fertile Window
Menstruation and ovulation are directly linked: a period is the result of an ovulation that didn’t lead to pregnancy. The fertile window, when conception is possible, spans about six days each cycle: the five days before ovulation and the day of ovulation itself.
A common belief is that ovulation always happens on day 14 of the cycle, but this is unreliable. Research tracking hundreds of cycles found that only 10% of women with 28-day cycles actually ovulated exactly 14 days before their next period. The timing ranged from day 10 to day 22. More than 70% of women had their fertile window fall outside the standard day-10-to-day-17 range that clinical guidelines once suggested. Even women with regular cycles show significant variation in when they ovulate.
When Periods Start and Stop
Menstruation begins at puberty, an event called menarche. The average age varies by region but generally falls between 12 and 13 years. In the United States, the median is around 12.5 years, while across Europe it ranges from about 12.6 in southern countries to over 15 in some regions. The age of first menstruation has been trending slightly younger over the past several decades.
Periods continue until menopause, which is defined as going 12 consecutive months without a menstrual cycle (not related to pregnancy, breastfeeding, or other conditions). Most women reach menopause between ages 45 and 55, with the average falling around age 48 to 49. Menopause before 40 is considered premature, and between 40 and 44 is classified as early.
Signs Your Period May Be Abnormal
Some variation is expected, but certain patterns suggest something worth investigating. Cycles shorter than 24 days or longer than 38 days, or periods that last more than 8 days, fall outside the normal range. Bleeding that varies by more than 20 days from cycle to cycle is considered irregular.
For bleeding volume, specific warning signs include soaking through a pad or tampon in two hours or less during most periods, passing large clots regularly, or needing to change products overnight. A history of being treated for anemia alongside heavy periods is another signal, as is excessive bleeding after dental work or minor procedures, which could point to an underlying bleeding disorder.
Managing Your Period
The most common menstrual products in higher-income countries are disposable pads and tampons. Reusable options include menstrual cups (small silicone cups inserted into the vagina to collect blood), commercially made reusable cloth pads, and period underwear with built-in absorbent layers. Each has tradeoffs in convenience, cost, and environmental impact. Menstrual cups and reusable products have a higher upfront cost but are significantly cheaper over time.
For cramps, anti-inflammatory painkillers reduce both pain and the prostaglandin levels driving it. Heat applied to the lower abdomen also relaxes uterine muscles. Hormonal contraceptives can lighten periods or stop them entirely, depending on the method, because they alter the hormonal cycle that triggers the lining buildup in the first place.