A period is your body’s way of resetting after a month of preparing for pregnancy. Each cycle, the uterus builds a thick, blood-rich lining designed to support a fertilized egg. When pregnancy doesn’t happen, hormone levels drop, and that lining sheds through the vagina. That shedding is your period.
Why the Body Sheds the Uterine Lining
The uterine lining, called the endometrium, has three jobs: prepare for a fertilized egg to implant, sustain a pregnancy if implantation happens, and shed itself when it doesn’t. The lining can’t simply stay in place cycle after cycle. It’s a specialized tissue packed with blood vessels and immune cells, actively maintained by hormones. When the hormonal signal to keep it drops, the tissue breaks down and exits the body.
What comes out during a period isn’t just blood. Menstrual fluid is a mix of blood, vaginal secretions, and cells from the uterine lining itself. It also contains immune cells, enzymes that help break down tissue, and signaling proteins. The typical amount lost is about 2 to 3 tablespoons of blood over the course of a period, though this varies widely from person to person.
How Hormones Drive Each Phase
The menstrual cycle runs on a feedback loop between the brain and the ovaries, driven primarily by two hormones: estrogen and progesterone. Each cycle averages about 28 days but can range from 21 to 35 days and still be considered normal. The cycle breaks into distinct phases, each with a specific role.
The menstrual phase is the period itself. Bleeding typically lasts three to five days, though anywhere from two to eight days falls within the normal range. During this time, the cervix opens slightly to allow menstrual fluid to pass through.
The follicular phase overlaps with menstruation and continues until ovulation. It lasts roughly 10 to 16 days. Estrogen rises during this window, signaling the uterus to start building a fresh lining. At the same time, follicles in the ovaries are maturing, and one will eventually release an egg.
The ovulatory phase is brief. The egg releases from the ovary around day 14 of a 28-day cycle, roughly 10 to 12 hours after a surge of a specific brain-released hormone. This is the window when pregnancy is possible.
The luteal phase is the most consistent part of the cycle, lasting about 14 days in nearly everyone. After the egg is released, the empty follicle produces progesterone. This hormone stabilizes and enriches the uterine lining, making it spongy and well-supplied with blood, essentially a hospitable environment for a fertilized egg. If no egg implants, progesterone drops sharply. That drop triggers the lining to break down, and menstruation begins again.
What Causes Cramps and Other Symptoms
Period cramps aren’t random. The uterus produces chemicals called prostaglandins that cause its muscular walls to contract and relax, squeezing the lining out. Higher levels of prostaglandins mean stronger contractions and more pain. This is why over-the-counter anti-inflammatory painkillers work well for cramps: they reduce the amount of prostaglandins the uterus makes, not just mask the pain.
Other common symptoms like bloating, breast tenderness, fatigue, and mood changes are tied to the hormonal shifts of the luteal phase. Progesterone affects fluid retention and can influence brain chemistry. These symptoms tend to peak in the days before bleeding starts and improve once the period begins.
What a Normal Period Looks Like
A “normal” period covers a wide range. Cycle length between 21 and 35 days, bleeding that lasts two to eight days, and flow that requires changing a pad or tampon every few hours all fall within healthy bounds. The color of menstrual fluid can range from bright red to dark brown, and small clots are common, especially on heavier days.
Some patterns are worth paying attention to. Soaking through a pad or tampon every hour for two or more consecutive hours is considered unusually heavy bleeding. Other signals that your cycle may need evaluation include periods that suddenly become much heavier or lighter, cycles shorter than 21 days or longer than 35, bleeding that lasts longer than eight days, or severe pain that doesn’t respond to standard painkillers. Losing more than about 5 tablespoons of blood per period (compared to the typical 2 to 3) is the general threshold for what’s clinically considered heavy.
Periods Across a Lifetime
Menstruation typically begins around age 13, though onset anywhere between 9 and 16 is common. The first few years of periods are often irregular as the hormonal system matures. Cycles may be longer, shorter, heavier, or lighter before settling into a more predictable pattern.
Periods continue until menopause, which occurs at a median age of 51. In the years leading up to menopause, a transition called perimenopause, cycles often become irregular again. Periods may come closer together or further apart, and flow can be unpredictable. This transition typically lasts several years before periods stop entirely. Once you’ve gone 12 consecutive months without a period, menopause is considered complete.
Over the course of a lifetime, this adds up to roughly 35 to 40 years of menstrual cycles, or somewhere around 400 to 450 periods total. Each one represents the same basic process: the body preparing for pregnancy, then clearing the slate to start again.