When you start your period, your uterus sheds the inner lining it spent the previous weeks building up. This process involves a coordinated cascade of hormone shifts, immune cell activity, and muscle contractions that typically lasts 2 to 7 days. Here’s what’s actually happening inside your body, what’s normal, and what to watch for.
The Hormone Drop That Starts Everything
Your period is set in motion about 14 days after ovulation, when the body recognizes that a fertilized egg hasn’t implanted. A small hormone-producing structure in the ovary called the corpus luteum breaks down, and levels of both progesterone and estrogen fall sharply. Progesterone, in particular, is what kept the uterine lining stable and thick. When it drops, that lining loses its support system and begins to break down.
This hormone withdrawal kicks off a two-phase process inside the uterus. The first phase is technically reversible: cells in the lining begin releasing inflammatory chemicals and prostaglandins in response to the falling progesterone. The second phase is not reversible. Immune cells flood into the tissue, and enzymes start dissolving the structural framework that holds the lining together. Once this phase begins, the shedding is happening whether progesterone returns or not.
How the Lining Actually Sheds
Specialized blood vessels in the uterine lining, called spiral arterioles, constrict tightly. This cuts off blood flow to the upper layer of the lining, creating a brief period of oxygen deprivation. That temporary lack of oxygen is a normal and necessary part of the process. It helps trigger tissue breakdown and, later, repair.
White blood cells, especially neutrophils, arrive in large numbers just before bleeding begins. They release enzymes that break down the tissue, separating the upper functional layer from the deeper base layer underneath. The base layer stays intact, which is important because it’s the foundation from which a new lining will grow.
Once the tissue detaches, a mix of blood, mucus, and endometrial fragments exits through the cervix, which sits lower and opens slightly during menstruation to let everything pass through.
What Menstrual Fluid Is Made Of
What comes out isn’t pure blood. Roughly half of menstrual fluid is blood, and the other half is a combination of cervical mucus, vaginal secretions, endometrial tissue, water, proteins, and dead cells. This is why menstrual fluid looks and behaves differently from blood you’d see from a cut. It’s often darker, thicker, and may not clot the same way.
Small clots are normal, especially on heavier days. They form when the flow is fast enough that the body’s natural anticoagulants can’t keep up. Clots become a concern only when they’re consistently as large as a quarter or bigger.
Why Cramps Happen
The same prostaglandins that help trigger lining breakdown also cause the muscular wall of the uterus to contract. These contractions squeeze the tissue and blood out through the cervix. When prostaglandin levels are higher, contractions are stronger, and cramps are more intense. This is the primary reason some people have mild periods while others experience significant pain.
Prostaglandins don’t stay neatly confined to the uterus. They can enter the bloodstream and affect other parts of the body, which is why your period can come with symptoms that seem unrelated to your reproductive system.
Symptoms Beyond Bleeding
Many people experience what’s sometimes called the “period flu,” a cluster of whole-body symptoms driven largely by prostaglandins circulating beyond the uterus. These can include diarrhea or loose stools (prostaglandins stimulate the smooth muscle in the intestines just like they do in the uterus), nausea, low-grade fever, fatigue, and generalized body aches.
Over-the-counter anti-inflammatory pain relievers work well for these symptoms precisely because they block prostaglandin production. Taking them at the first sign of cramps or even slightly before your period starts is more effective than waiting until the pain is established, because it prevents prostaglandins from building up in the first place.
How Much Bleeding Is Typical
Most periods involve a total loss of about 30 to 80 milliliters of blood, spread over the full duration. That’s roughly 2 to 5 tablespoons. Flow is usually heaviest in the first two or three days, then tapers off. The entire period typically lasts between 2 and 7 days, with cycles repeating every 21 to 35 days.
Signs that bleeding has crossed into “heavy” territory include soaking through a pad or tampon every hour for several consecutive hours, needing to double up on pads, having to change protection overnight, bleeding that stretches beyond 7 days, or passing clots the size of a quarter or larger. Any of these patterns is worth bringing up with a healthcare provider, since chronic heavy periods can lead to iron deficiency over time.
How the Lining Repairs Itself
One of the most remarkable parts of menstruation is how quickly the uterus heals. Within 3 to 5 days of shedding, the lining is already rebuilt. New cells grow outward from the base of uterine glands, spreading across the raw surface until they meet cells migrating from neighboring glands. This creates a fresh inner surface, and it heals without scarring, something that’s unusual in the body. It’s this rapid, scar-free repair cycle that allows the process to repeat month after month without cumulative damage.
Signs of a First Period
If you haven’t had a period yet and are wondering when it might arrive, there are some reliable physical markers. Breast development usually comes first. Periods typically start about two years after breasts begin growing. Growth of underarm and pubic hair is another signal that menstruation is approaching. The first period, called menarche, often starts light and irregular. It can take a year or two for cycles to settle into a predictable pattern, because the hormonal feedback loop between the brain and ovaries is still calibrating.
First periods sometimes show up as brownish spotting rather than bright red bleeding, which can be confusing if you’re expecting something more dramatic. A thin pantyliner is usually enough for the first day or two while you figure out what your flow looks like.