A period typically lasts 4 to 5 days, though anywhere from 2 to 7 days falls within the normal range. The reason it takes multiple days rather than happening all at once comes down to how the uterine lining breaks down: it doesn’t shed like a sheet peeling off a wall. Instead, it’s a gradual, region-by-region process involving inflammation, tissue breakdown, blood vessel cycling, and simultaneous repair, all orchestrated by shifting hormone levels.
What Triggers the Shedding
The whole process starts with a drop in progesterone. During the second half of your cycle, progesterone keeps the uterine lining thick and stable, ready to support a potential pregnancy. When no fertilized egg implants, the structure that produces progesterone (a temporary gland on the ovary) breaks down, and progesterone levels plummet. That withdrawal is the signal for menstruation to begin.
The falling progesterone sets off a chain reaction. Inflammatory chemicals flood the lining, white blood cells infiltrate the tissue, and specialized enzymes begin dissolving the structural scaffolding that holds the lining together. This is essentially a controlled injury. The uterus is systematically dismantling living tissue, and that kind of process takes time to complete safely.
Why It Can’t Happen All at Once
The lining doesn’t break down uniformly. Different regions of the uterus shed at different times, with tissue destruction happening in waves across the surface. While one area is actively shedding, another may already be starting to heal. This staggered pattern is a big part of why bleeding stretches over several days rather than finishing in a few hours.
The blood vessels in the lining, called spiral arteries, also cycle through phases of constriction and relaxation. They clamp down to limit blood loss, then open again, releasing more blood and tissue. This rhythmic on-off pattern controls the rate of bleeding so the body doesn’t lose too much blood at once. Total blood loss across an entire period is surprisingly small: about 2 to 3 tablespoons for most people.
Chemical messengers called prostaglandins play a central role in pacing the process. They cause the uterine muscles and blood vessels to contract, helping push tissue out. Prostaglandin levels are highest on the first day of your period (which is also why cramps tend to be worst at the start) and gradually decline as more of the lining is shed. That tapering is why flow is usually heaviest in the first day or two and lighter toward the end.
Repair Starts Before Shedding Finishes
One of the more remarkable aspects of menstruation is that the uterus begins repairing itself while it’s still shedding. New surface cells appear to arise from small stem-like cells embedded in the deeper layer of the uterine wall, a layer that stays intact throughout menstruation. These new cells don’t even rely on the usual process of cell division to regenerate the surface. Instead, they differentiate rapidly from stromal cells that act as progenitor cells, restoring the lining at a speed that’s unusual for human tissue repair.
This overlap between destruction and reconstruction is another reason the process takes days. The body is essentially running two opposing operations simultaneously, balancing how fast tissue is shed against how fast it can be rebuilt. The lining can’t be expelled faster than the repair mechanisms can keep up, or bleeding would become uncontrolled.
What Makes Periods Shorter or Longer
The 4-to-5-day average is just that: an average. Plenty of factors shift where you fall in the 2-to-7-day range. Prostaglandin levels vary between individuals, which affects how efficiently the uterus contracts and expels tissue. A thicker lining (built up from higher estrogen levels) simply takes longer to shed. Even the physical size and shape of the uterus can influence duration.
Your life stage matters too. During the perimenopausal transition, which can start in the early 40s, estrogen levels become unpredictable. Periods may grow longer or shorter, heavier or lighter, as ovulation becomes less consistent. When ovulation doesn’t happen in a given cycle, progesterone isn’t produced in normal amounts, and the lining can build up excessively before shedding in a prolonged, heavy bleed.
When a Week-Long Period Signals Something Else
Bleeding that regularly lasts longer than 7 days, or that requires you to change a pad or tampon every hour for several consecutive hours, crosses into what’s medically considered heavy menstrual bleeding. Several conditions can push periods past the normal range.
- Fibroids and polyps. Noncancerous growths in or on the uterus increase the surface area that bleeds and can interfere with the uterus’s ability to contract and stop bleeding.
- Hormonal imbalances. Conditions like PCOS, thyroid disorders, obesity, and insulin resistance can disrupt the balance between estrogen and progesterone. Without enough progesterone, the lining grows too thick and sheds irregularly.
- Adenomyosis. Tissue similar to the uterine lining grows into the muscular wall of the uterus, causing heavier and longer bleeding.
- Bleeding disorders. Conditions like von Willebrand’s disease impair the blood’s ability to clot, which can extend bleeding duration.
- Medications. Blood thinners and certain hormonal medications can increase how long and how heavily you bleed.
A period that consistently hits the 7-day mark and has always done so is likely just your normal pattern. A period that suddenly starts lasting longer than it used to, or that pushes past 8 days, is worth investigating. The distinction matters because prolonged heavy bleeding can lead to iron deficiency over time, causing fatigue and other symptoms that are easy to dismiss as unrelated.