A period is the monthly shedding of the lining inside the uterus. When the body prepares for pregnancy each month and pregnancy doesn’t happen, the thickened uterine lining breaks down and leaves the body as blood and tissue through the vagina. Most periods last three to five days, though anywhere from two to seven days is normal. A typical period involves losing about two to three tablespoons of blood total, which can look like more than it is because it’s mixed with tissue and mucus.
Why Periods Happen
Each month, the body runs through a cycle designed to prepare for a possible pregnancy. Two hormones, estrogen and progesterone, signal the lining of the uterus (called the endometrium) to thicken and fill with blood vessels so it could support a fertilized egg. When no egg is fertilized, estrogen and progesterone levels drop sharply. Without those hormones sustaining it, the thickened lining has no reason to stay. It breaks apart and flows out through the vagina as a period.
This isn’t a one-time reset. The cycle starts over immediately, with the body already beginning to build a new lining while bleeding is still happening.
The Four Phases of the Cycle
A full menstrual cycle runs from the first day of one period to the first day of the next. That’s typically 21 to 35 days, with 28 days being a common average. The cycle has four distinct phases, each driven by different hormone shifts.
Menses Phase
This is the period itself. Bleeding usually lasts three to five days. The uterine lining, blood, and mucus leave the body through the vagina. Hormone levels are at their lowest point during this phase.
Follicular Phase
This phase overlaps with the period and continues after bleeding stops. Estrogen levels rise, causing the uterine lining to start thickening again. At the same time, small fluid-filled sacs in the ovaries (called follicles) begin developing, each containing an egg. Usually one follicle becomes dominant and continues maturing while the others stop growing.
Ovulation
Around day 14 of a 28-day cycle, a surge of hormones triggers the ovary to release a mature egg. This is the point in the cycle when pregnancy is possible. The egg travels down the fallopian tube toward the uterus. It survives for about 12 to 24 hours after release.
Luteal Phase
This phase runs from roughly day 15 to day 28. Progesterone rises to maintain the thickened uterine lining. If the egg isn’t fertilized, progesterone drops, the lining becomes unstable, and the cycle loops back to a period.
When Periods Start
The first period, called menarche, happens during puberty. Half of girls in the United States get their first period by age 11 years and 10 months, based on CDC data from 2013 to 2017. About 10% start by age 10, and 90% have started by age 14. The median age has shifted slightly younger over the past few decades, dropping from 12.1 years in 1995 to 11.9 years in recent data.
Early periods are often unpredictable. Cycles can range anywhere from 21 to 45 days apart, and some girls skip months entirely. This happens because the hormonal system controlling the cycle is still maturing. By the third year after the first period, 60 to 80% of cycles settle into the adult pattern of 21 to 34 days.
What Cramps and Other Symptoms Feel Like
Period cramps are caused by the uterus contracting to push out its lining. These contractions are triggered by chemicals called prostaglandins, which also play a role in pain and inflammation. Girls and women who produce higher levels of prostaglandins tend to have more intense cramps.
Cramping pain usually starts one to three days before the period begins, peaks about 24 hours after bleeding starts, and fades within two to three days. It’s typically felt as a throbbing or aching sensation in the lower abdomen that can spread to the lower back and thighs. Some people also experience nausea, loose stools, headaches, or dizziness during their period.
Mood changes, bloating, breast tenderness, and fatigue are also common in the days leading up to a period. These symptoms, often grouped under the term PMS, are driven by the hormone shifts happening during the luteal phase.
Period Products and How to Use Them
There are several types of products designed to absorb or collect menstrual blood:
- Pads stick to the inside of underwear and absorb blood externally. They should be changed every few hours, or more often on heavier days.
- Tampons are inserted into the vagina and absorb blood internally. They should be changed every four to eight hours, and you should always use the lowest absorbency that works for your flow. Never leave a tampon in for more than eight hours.
- Menstrual cups are small flexible cups inserted into the vagina to collect blood rather than absorb it. They’re reusable and should be cleaned daily during use, then sanitized in boiling water for one to two minutes after each cycle ends.
- Menstrual discs work similarly to cups but sit in a different position inside the vagina.
- Period underwear has built-in absorbent layers and can be worn alone or as backup with another product.
Many people start with pads because they’re the simplest to use, then try other options as they get more comfortable. There’s no single “right” product, and it’s common to use different types depending on the day or activity.
Signs of a Heavy or Unusual Period
A typical period lasts about four to five days and involves relatively little blood loss. A period is considered heavy if it lasts longer than seven days, requires changing a pad or tampon more often than every two hours, or involves blood clots the size of a quarter or larger. Needing to double up on pads or waking up at night to change products are also signs of unusually heavy flow.
Heavy periods can lead to fatigue, low energy, and shortness of breath over time because of iron lost through excess bleeding. Persistent pain in the lower abdomen during every period, or bleeding that regularly interferes with normal activities, is also worth paying attention to. These patterns don’t always signal a serious problem, but they’re not something to just push through either.