A period is the monthly shedding of the uterine lining, released from the body as a mix of blood, tissue, and fluid through the vagina. It happens when a pregnancy has not occurred that cycle, signaling the body to clear the lining it built up and start fresh. Most people with a uterus experience periods from their early teens until sometime between ages 45 and 55, when menopause begins.
Why It Happens
Each month, the body prepares for a potential pregnancy. The lining of the uterus (called the endometrium) thickens with fluids and nutrients designed to support a fertilized egg. Two hormones drive this process: estrogen and progesterone. They rise through the cycle, stimulating the uterine lining to grow.
If no egg is fertilized, or if a fertilized egg doesn’t implant in the uterine wall, a structure in the ovary called the corpus luteum breaks down after about 14 days. This causes estrogen and progesterone levels to drop sharply. Without progesterone to maintain it, the thickened lining stops developing and begins to shed. That shedding is what comes out as a period, and it marks the start of a new menstrual cycle.
What Menstrual Fluid Actually Contains
Period fluid is not purely blood. It’s a combination of blood, fragments of endometrial tissue that lined the uterus, and vaginal secretions. This is why it often looks different from blood you’d see from a cut: it can be darker, thicker, or contain small clots or tissue. The color can range from bright red to dark brown, especially toward the beginning and end of a period.
Most periods involve less blood than people expect. The total blood volume is typically under 45 milliliters across the entire period, roughly three tablespoons. A period is considered moderately heavy at 60 to 100 milliliters and excessive above 100 milliliters.
How Long a Period Lasts
Bleeding typically lasts between three and seven days. The full menstrual cycle, measured from the first day of one period to the first day of the next, normally falls between 21 and 35 days, with 28 days being a common average. These numbers vary from person to person and can even shift from month to month in the same person.
Periods usually start during the early teen years (a first period is called menarche) and continue until menopause, which occurs between ages 45 and 55 for most women worldwide.
What Cramps and Symptoms Feel Like
During a period, the uterus contracts to push out its lining. These contractions are triggered by hormone-like substances called prostaglandins, which are involved in pain and inflammation. Higher prostaglandin levels lead to stronger contractions and more intense cramps. The pain is usually felt in the lower abdomen and can radiate into the lower back or thighs.
Cramps are the most recognizable symptom, but the hormonal shifts surrounding a period can cause a wide range of physical effects: bloating, breast tenderness, headaches, fatigue, acne flare-ups, joint or muscle pain, and changes in digestion like constipation or diarrhea. Weight can fluctuate slightly due to fluid retention.
Emotional and behavioral changes are also common in the days leading up to a period, a window often referred to as PMS (premenstrual syndrome). These can include mood swings, irritability, anxiety, difficulty concentrating, trouble sleeping, food cravings, and feeling withdrawn. Not everyone experiences PMS, and severity varies widely. For some it’s barely noticeable; for others it significantly disrupts daily life.
What Counts as Irregular
Some variation between cycles is normal, but certain patterns fall outside the typical range and can signal an underlying issue. A period is generally considered irregular if:
- Cycles are shorter than 21 days or longer than 35 days apart
- The gap between cycles varies by more than nine days (for example, 28 days one month, then 37, then 29)
- Bleeding lasts longer than seven days
- Flow is much heavier or lighter than usual
- You miss three or more periods in a row without being pregnant, breastfeeding, or approaching menopause
Severe pain with nausea or vomiting during a period, bleeding between periods, and soaking through a pad or tampon in an hour are also considered abnormal and worth discussing with a healthcare provider. The absence of any period for 90 days or more is clinically significant unless pregnancy, breastfeeding, or menopause explains it.
Managing Period Symptoms
For cramps, over-the-counter pain relievers that target inflammation tend to work best because they reduce prostaglandin production. Taking them at the first sign of cramping, rather than waiting until pain peaks, is more effective. Heat applied to the lower abdomen (a heating pad or hot water bottle) also relaxes the uterine muscles and can ease discomfort.
For managing flow, options include pads, tampons, menstrual cups, and period underwear. Each has trade-offs in comfort, convenience, and cost, and many people use a combination depending on their flow that day. Lighter days at the start or end of a period might only need a liner, while heavier days in the middle may require more absorbent products or more frequent changes.
Staying hydrated, getting regular physical activity, and reducing salt intake in the days before a period can help with bloating and fluid retention. Exercise also tends to reduce cramp severity for many people, even though it might be the last thing that feels appealing. For PMS-related mood symptoms, consistent sleep and limiting caffeine and alcohol in the premenstrual window can make a noticeable difference.