Your period is the shedding of your uterine lining, released as blood and tissue through the vagina. It happens roughly once a month when pregnancy hasn’t occurred, and it’s the most visible part of a larger hormonal cycle that your body runs continuously from puberty until menopause. A typical period lasts 2 to 7 days, with most people losing about 60 milliliters (around 2.7 ounces) of blood total.
What Happens Inside Your Body
Each month, the lining of your uterus (called the endometrium) thickens with blood-rich tissue in preparation for a fertilized egg. If no egg implants, a structure in the ovary called the corpus luteum breaks down, and your progesterone levels drop sharply. That drop triggers an inflammatory response in the uterine lining: immune cells flood in, enzymes break down the tissue, and the upper two-thirds of the lining detaches and exits through the cervix and vagina. That’s your period.
The process is surprisingly demanding. Your body essentially wounds and then completely repairs the inner surface of the uterus every single cycle. This monthly regeneration is unusual in the animal kingdom, and it gives the human uterus a remarkable ability to adapt and rebuild.
The Four Phases of Your Cycle
Your period is just one phase of a cycle that typically runs 24 to 38 days from the first day of one period to the first day of the next. Four hormones orchestrate the whole thing: estrogen, progesterone, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).
Menstrual Phase (Days 1 to 7)
This is your period. Estrogen and progesterone are at their lowest, and the uterine lining sheds. FSH starts rising toward the end of this phase, signaling the ovaries to begin preparing a new egg.
Follicular Phase (Days 1 to 13)
Overlapping with menstruation, this phase begins on day one and runs until ovulation. FSH prompts several follicles in the ovary to start developing, though usually only one matures fully. As the follicle grows, it produces rising amounts of estrogen, which thickens the uterine lining again. By the end of this phase, your body’s daily estrogen production has increased roughly tenfold compared to the start.
Ovulation (Around Day 14)
When estrogen reaches a critical threshold, it triggers a surge of LH. This surge begins about 34 to 36 hours before ovulation and causes the mature follicle to release its egg into the fallopian tube. This is the fertile window, and it’s the shortest phase, lasting roughly 24 hours.
Luteal Phase (Days 15 to 28)
The empty follicle transforms into the corpus luteum, which pumps out progesterone. Daily progesterone production jumps from about 1 milligram during the early follicular phase to around 25 milligrams at its peak. Progesterone stabilizes and enriches the uterine lining, making it hospitable for a potential embryo. If no pregnancy occurs, the corpus luteum degenerates after about 10 to 14 days, progesterone plummets, and the cycle starts over with your next period.
Why Periods Hurt
Cramps are the most common period symptom, and they have a straightforward cause. To shed its lining, your uterus contracts. Those contractions are driven by prostaglandins, hormone-like compounds involved in pain and inflammation. The more prostaglandins your body produces, the stronger the contractions and the worse the cramps. This is why anti-inflammatory pain relievers, which block prostaglandin production, tend to work well for period pain.
Prostaglandins don’t stay confined to the uterus, though. They circulate through your body and can cause nausea, loose stools, headaches, and dizziness. These symptoms are most common on the first one or two days of bleeding, when prostaglandin levels peak. Fatigue and bloating, driven more by hormonal shifts than prostaglandins, often begin a few days before bleeding starts.
What Counts as Normal
There’s a wide range of normal when it comes to periods. Here are the clinical benchmarks:
- Cycle length: 24 to 38 days, measured from the first day of one period to the first day of the next
- Bleeding duration: 2 to 7 days
- Blood loss: 5 to 80 milliliters per cycle, with an average around 60 milliliters
- Regularity: Your cycle can vary by up to 7 days from month to month and still be considered regular
Cycles shorter than 24 days, longer than 38 days, or that vary by more than 20 days between cycles fall outside the normal range. Bleeding that lasts more than 8 days or consistently exceeds 80 milliliters (soaking through a pad or tampon every two hours) is classified as heavy menstrual bleeding and is worth investigating.
When Your Period Changes
Most people get their first period between ages 12 and 14, though it can start as early as 9 or as late as 16. The first couple of years often bring irregular cycles as the hormonal system calibrates. Cycles typically become more predictable by the late teens.
At the other end, menopause is defined as going 12 consecutive months without a period, unrelated to pregnancy or breastfeeding. The average age is around 51, but it can range from the early 40s to the late 50s. In the years leading up to menopause (perimenopause), cycles often become irregular, with skipped periods, heavier flows, or shorter cycles.
The American College of Obstetricians and Gynecologists considers the menstrual cycle a vital sign, on par with blood pressure and heart rate. Changes in your cycle pattern can signal conditions like thyroid disorders, polycystic ovary syndrome, or hormonal imbalances, sometimes years before other symptoms appear. Tracking your cycle length, flow, and symptoms gives you and any healthcare provider a useful baseline.
Managing Your Period: Product Capacity
Choosing a menstrual product is partly about comfort and partly about matching the product’s capacity to your flow. A 2024 study that tested 21 products using real blood (rather than saline, which products absorb differently) found significant differences in how much each type holds:
- Menstrual discs: 40 to 80 milliliters, the highest capacity of any product category
- Menstrual cups: 22 to 35 milliliters, depending on size
- Tampons: 20 milliliters for regular, 31 to 34 milliliters for heavy-flow varieties
- Pads (heavy/ultra): 31 to 52 milliliters, varying widely by brand
- Light pads or liners: about 4 milliliters
- Period underwear: 1 to 3 milliliters, far less than most people expect
Since the average total blood loss per period is around 60 milliliters spread over several days, a regular tampon or medium pad is enough for most people on lighter days, while cups or discs offer the longest wear time on heavy days. Period underwear works best as backup protection rather than a standalone product, given its low measured capacity.
Signs Something May Be Off
Some period changes are worth paying attention to. Cycles that consistently fall outside the 24-to-38-day range, bleeding that lasts longer than 8 days, or flow so heavy you’re soaking through a pad or tampon every hour can all point to underlying conditions. A history of gushing blood, needing to change protection every two hours, or periods that have led to anemia may indicate a bleeding disorder, especially in teenagers whose cycles are still establishing.
Periods that suddenly stop for three months or more (outside of pregnancy or breastfeeding), severe pain that doesn’t respond to over-the-counter pain relief, or bleeding between periods are also worth discussing with a healthcare provider. These patterns don’t always mean something serious, but they’re the kind of signal the menstrual cycle was designed to give you.