A period is the shedding of the lining of the uterus, resulting in bleeding from the vagina that typically lasts 2 to 7 days. It happens roughly once a month as part of the menstrual cycle, the body’s recurring process of preparing for a possible pregnancy. When pregnancy doesn’t occur, the thickened uterine lining breaks down and exits the body as a mix of blood, tissue, and fluid.
Why Bleeding Happens
Each month, the body builds up a thick, blood-rich lining inside the uterus in case a fertilized egg needs a place to implant and grow. Two hormones drive this process: estrogen thickens the lining during the first half of the cycle, and progesterone stabilizes it during the second half.
If no pregnancy occurs, progesterone levels drop sharply. That drop is the direct trigger for a period. Without progesterone’s stabilizing effect, the blood vessels supplying the upper layer of the uterine lining coil tightly and constrict, cutting off blood flow to the tissue. The body then launches a localized inflammatory response: immune cells flood into the lining, and enzymes break down the tissue’s structural framework. The upper two-thirds of the lining detaches and sheds, leaving the deeper base layer intact so the lining can rebuild during the next cycle.
The Four Phases of the Cycle
A period is just one part of a larger hormonal cycle. The full menstrual cycle runs from the first day of one period to the first day of the next and is typically 24 to 38 days long. It has four distinct phases:
- Menstrual phase (days 1 to ~5): The lining sheds. This is the period itself.
- Follicular phase (overlapping with menstruation, ending at ovulation): Estrogen rises, rebuilding the uterine lining. In the ovaries, a small fluid-filled sac called a follicle matures and prepares to release an egg.
- Ovulation (around day 14 in a 28-day cycle): The mature egg is released from the ovary and travels into the fallopian tube. This is the window when pregnancy can occur.
- Luteal phase (ovulation to the next period): Progesterone rises to maintain the thickened lining. If no fertilized egg implants, progesterone falls and the cycle starts over.
Not everyone runs on a textbook 28-day schedule. Cycles anywhere from 21 to 35 days are considered normal, and some variation from month to month is common, especially during the first few years of menstruation and again in the years approaching menopause.
How Much Blood Is Normal
The total amount of blood lost during a single period is usually around 60 milliliters, or roughly 4 tablespoons. That can feel like more than it is because the fluid also contains tissue and mucus. The normal range spans 5 to 80 milliliters across the full period, with heavier flow on the first two days tapering off toward the end.
Measuring exact volume at home isn’t practical, so what matters more is your experience. Signs that bleeding may be heavier than normal include soaking through a pad or tampon every two hours, passing blood clots larger than a quarter, needing to change products overnight, or feeling that bleeding interferes with daily activities. Regularly losing more than 80 milliliters per cycle is considered heavy menstrual bleeding and is worth discussing with a healthcare provider, especially if it leaves you feeling exhausted or lightheaded.
Common Physical and Emotional Symptoms
A period often comes with symptoms beyond bleeding. Many people experience a cluster of physical and emotional changes in the days leading up to and during their period, commonly called premenstrual syndrome (PMS). These symptoms are driven by the same hormonal shifts that trigger the period itself.
The most common physical symptom is abdominal cramping, reported by roughly 63% of people who menstruate. Cramps happen because the uterus contracts to help expel the lining, and the same inflammatory chemicals (prostaglandins) that break down the tissue also trigger pain. Fatigue affects about a third of people, followed by bloating, back pain, breast tenderness, acne, and stomach upset.
On the emotional side, irritability is the most frequently reported symptom (around 40%), followed by anxiety (35%), low mood (23%), and stress (18%). Some people also notice food cravings or changes in appetite. These symptoms range from mild and barely noticeable to severe enough to disrupt daily life. For most people, they ease within a day or two of the period starting as hormone levels stabilize.
When Periods Start and Stop
Most people get their first period, called menarche, around age 13, though anywhere between 9 and 16 is typical. Cycles are often irregular for the first year or two as the body’s hormonal signaling system matures.
Periods continue until menopause, which occurs at a median age of 51. In the years leading up to menopause (a phase called perimenopause), cycles often become irregular, with periods arriving closer together or further apart, sometimes with heavier or lighter flow than usual. Menopause is confirmed after 12 consecutive months without a period.
Managing Your Period
Several types of products exist for absorbing or collecting menstrual fluid. External options include sanitary pads (worn inside underwear) and period underwear (built-in absorbent layers). Internal options include tampons (absorbent material inserted into the vagina), menstrual cups (reusable silicone cups that collect fluid), and menstrual discs (similar to cups but sit in a different position). The choice comes down to comfort, activity level, flow heaviness, and personal preference. Many people use different products on different days of their period as flow changes.
For managing cramps and discomfort, over-the-counter anti-inflammatory pain relievers work by blocking the same prostaglandins that cause uterine contractions and pain. Heat applied to the lower abdomen or back also helps relax the uterine muscle. Staying physically active, even lightly, can reduce cramping for some people.
Signs of an Irregular Period
Cycles shorter than 24 days or longer than 38 days fall outside the normal range. Bleeding that lasts more than 8 days, cycles that vary by more than 20 days from one month to the next, or periods that suddenly stop for several months (outside of pregnancy) are also considered irregular. Light spotting between periods or bleeding after sex can occur for benign reasons but should be evaluated if it happens repeatedly.
Tracking your cycle for a few months, even with a simple calendar, gives you a baseline that makes it easier to spot changes. Noting the start date, end date, and general flow level is enough to identify patterns worth bringing up at a checkup.