A menstrual period is the bleeding that happens when the lining of the uterus sheds through the vagina, typically lasting 4 to 5 days with about 2 to 3 tablespoons of total blood loss. It’s one part of a repeating monthly cycle in which the body prepares for a possible pregnancy, then resets when pregnancy doesn’t occur. Periods usually begin around age 12 and continue until menopause, which happens on average around age 51.
How the Menstrual Cycle Works
A menstrual cycle is counted from the first day of one period to the first day of the next. While 28 days is often cited as the “standard,” cycles anywhere from 21 to 35 days are normal for adults. The cycle has three main phases, each driven by shifting hormone levels.
The first phase, the follicular phase, begins on day one of your period and lasts about 13 to 14 days. During this time, a hormone called FSH signals the ovaries to develop a follicle containing an egg. As the follicle grows, estrogen levels rise steadily, which triggers the uterine lining to thicken with blood vessels and tissue. By about day 7, estrogen is high enough to start slowing FSH production, so typically only one follicle continues maturing.
Next comes ovulation. When estrogen reaches a critical level, usually around day 13, the brain releases a surge of another hormone (LH) that causes the mature egg to release from the ovary. The ovulation window itself is brief, lasting only 16 to 32 hours. This is the most fertile point in the cycle. Ovulation consistently happens about 14 days before your next period starts, regardless of how long your overall cycle is. So if your cycle is 26 days, you likely ovulate around day 12; if it’s 30 days, around day 16.
The final phase, the luteal phase, lasts about 14 days. After releasing the egg, the leftover follicle transforms into a structure that pumps out progesterone. This hormone makes the uterine lining even more lush and blood-rich, creating an ideal environment for a fertilized egg to implant. If no pregnancy occurs, progesterone and estrogen both drop sharply. Without hormonal support, the thickened lining breaks down, and the cycle resets with a new period.
What a Period Feels Like
Bleeding typically lasts 4 to 5 days, though anywhere from 2 to 7 days falls within the normal range. Flow is usually heaviest in the first two days and gradually tapers off. The blood can range from bright red to dark brown, and it’s common to see small clots.
Cramps are the most recognizable symptom. During your period, the uterus contracts to help push out its lining. These contractions are triggered by hormone-like chemicals called prostaglandins. Higher levels of prostaglandins mean stronger contractions and more pain. Cramping typically starts 1 to 3 days before bleeding begins, peaks about 24 hours into your period, and subsides within 2 to 3 days. The pain centers in the lower abdomen but often radiates to the lower back and thighs.
Beyond cramps, many people experience nausea, loose stools, headaches, and dizziness during their period. These symptoms are also linked to prostaglandins and hormonal shifts, not just to the uterus itself. Prostaglandins can affect the digestive tract, which is why bowel changes around your period are so common.
When Periods Start and Stop
The first period, called menarche, now occurs at a median age of about 11 years and 10 months in the United States. That average has been slowly declining: it was 12.1 years in 1995 and dropped to 11.9 by 2013 to 2017. About 10% of girls reach menarche by age 10, roughly half by age 12, and 90% by age 14. It’s normal for cycles to be irregular for the first year or two as the hormonal system matures.
Periods continue until menopause, which typically occurs between ages 45 and 55. In the years leading up to menopause (perimenopause), cycles often become irregular, with periods arriving closer together or farther apart, and flow becoming lighter or heavier before stopping entirely.
Managing Period Products
The most commonly used products are pads (sanitary napkins) and regular tampons, followed by panty liners and super-absorbent tampons. A smaller but growing number of people use menstrual cups, which are flexible silicone cups inserted into the vagina to collect blood, or period underwear, which has absorbent material built into the fabric.
Which product works best depends on your flow, comfort, and lifestyle. Pads are external and require no insertion. Tampons and menstrual cups are worn internally and tend to be preferred during physical activity or swimming. Menstrual cups are reusable, which makes them more cost-effective over time and produces less waste. Many people use a combination of products, switching between tampons during the day and pads at night, for instance.
Irregular or Heavy Periods
Not every cycle runs like clockwork, and occasional variation is normal. But consistently irregular periods can signal an underlying issue. Common causes of lighter or missing periods include thyroid problems, high stress hormone levels, eating disorders, excessive exercise, uncontrolled diabetes, and hormonal birth control. Pregnancy is always the first thing to rule out when a period is late or absent.
On the other end, unusually heavy or prolonged bleeding has its own set of causes. Polycystic ovary syndrome (PCOS), uterine fibroids (noncancerous growths in the uterine wall), polyps on the uterine lining, and bleeding disorders can all increase menstrual flow significantly. If you’re soaking through a pad or tampon every hour for several consecutive hours, or if your period regularly lasts longer than 7 days, that’s worth investigating with a healthcare provider.
Severe menstrual pain also has specific causes beyond normal prostaglandin activity. Endometriosis, where tissue similar to the uterine lining grows outside the uterus, is one of the most common. Structural abnormalities in the uterus and certain types of IUDs can also intensify cramping. Pain that interferes with daily activities or doesn’t respond to over-the-counter pain relief is not something you have to accept as normal.
Ovulation, Fertility, and the Period Connection
Your period and your fertility are directly linked. Ovulation happens about 14 days before your period starts, and an egg survives only about 12 to 24 hours after release. Sperm, however, can live in the reproductive tract for up to 5 days. This means the fertile window opens several days before ovulation and closes shortly after it. For someone with a 26- to 28-day cycle, ovulation falls between days 12 and 14.
If you’re trying to conceive, tracking your cycle length helps estimate when ovulation is likely. If you’re trying to avoid pregnancy, keep in mind that the follicular phase (the time between your period and ovulation) is the most variable part of the cycle, which makes predicting ovulation from the start of your period less reliable for people with irregular cycles. The luteal phase, by contrast, stays relatively fixed at about 14 days.