During a period, the lining of your uterus breaks down and leaves your body through the vagina, along with blood, mucus, and vaginal secretions. This process typically lasts four to six days, though anything from two to eight days falls within the normal range. What you experience during those days involves a coordinated sequence of hormonal shifts, muscle contractions, and chemical signals that affect far more than just your uterus.
Why Bleeding Starts
The entire process is triggered by a drop in progesterone. Throughout the second half of your cycle, progesterone keeps the uterine lining thick and stable, preparing for a possible pregnancy. When no fertilized egg implants, the tissue that produces progesterone breaks down, and levels plummet. This withdrawal is the direct trigger for the lining to start shedding.
Once progesterone drops, enzymes begin breaking apart the structural proteins holding the uterine lining together. The blood vessels that had been feeding this tissue constrict and then rupture, releasing blood into the uterine cavity. The lining detaches in fragments rather than peeling off all at once, which is why bleeding happens over several days instead of in a single event. Your body simultaneously begins repairing the raw surface even while shedding is still underway.
What Menstrual Fluid Actually Contains
What comes out during a period isn’t purely blood. Menstrual fluid is a mix of blood, cells from the uterine lining, vaginal secretions, and mucus. The blood component is why it looks red or dark brown, but the tissue fragments and mucus give it a thicker, sometimes clumpy texture that differs from a cut on your finger.
The average total volume over the entire period is about 80 to 87 mL, roughly a third of a cup. Some people lose as little as 15 mL, while others lose over 160 mL. Flow tends to be heaviest in the first two or three days, then gradually tapers off. Small clots are normal and form when blood pools in the uterus before leaving the body. If you’re regularly passing clots larger than a quarter or soaking through a pad every hour for several hours in a row, that’s worth discussing with a doctor.
What Causes Cramps
Period cramps come from your uterus physically contracting to push the lining out. These contractions are driven by chemicals called prostaglandins, which are produced in the uterine lining itself. Higher concentrations of prostaglandins mean stronger contractions and more pain. This is why the first day or two of a period, when prostaglandin levels peak, tends to be the most uncomfortable.
The contractions temporarily compress the blood vessels supplying the uterus, briefly cutting off oxygen to the muscle. That oxygen deprivation is what produces the deep, aching pain characteristic of period cramps. It’s the same basic mechanism that causes a muscle cramp anywhere else in your body. Some people produce more prostaglandins than others, which is why cramp severity varies so much from person to person.
Why Your Digestion Changes
Prostaglandins don’t stay neatly contained in the uterus. They circulate through your bloodstream and act on smooth muscle tissue throughout your body, including in your intestines. When prostaglandins reach the muscles lining your colon, they can speed up contractions there too, leading to looser stools, more frequent bowel movements, or diarrhea during the first few days of your period.
Some people experience the opposite: bloating and constipation in the days just before their period starts, followed by the prostaglandin-driven shift once bleeding begins. Nausea can also occur when prostaglandin levels are high. These digestive symptoms are not a sign that something is wrong. They’re a predictable side effect of the same chemicals causing your uterus to contract.
Hormones at Their Lowest Point
During your period (roughly days one through five of a 28-day cycle), both estrogen and progesterone sit at their lowest levels. This hormonal low point is the reason many people feel different physically and emotionally during menstruation compared to other parts of their cycle.
Low estrogen pulls down serotonin activity in the brain. Serotonin plays a central role in mood stability, sleep, and appetite, so when it dips, you may feel more irritable, tired, or emotionally sensitive. Low progesterone also reduces levels of a calming brain chemical called allopregnanolone, which normally has an anxiety-reducing effect. The combination of these neurochemical shifts explains why the days surrounding your period can feel emotionally heavier, even when nothing in your life has changed. For most people, mood lifts noticeably once estrogen starts climbing again in the days after bleeding ends.
Other Physical Changes
Your cervix shifts position during menstruation. It sits lower in the vaginal canal than it does at other points in your cycle and feels firmer to the touch. It also opens slightly to allow menstrual fluid to pass through. These changes are subtle, and most people never notice them unless they’re specifically checking.
Fatigue is common during your period, partly from the hormonal low point and partly from the physical demands of shedding and rebuilding the uterine lining. Iron leaves your body with menstrual blood, and people with heavier periods can develop low iron stores over time, which compounds the tiredness. Headaches, breast tenderness that lingers from premenstrual days, and mild lower back pain from uterine contractions radiating to surrounding muscles are all typical.
Body temperature also shifts. The slight rise in basal body temperature caused by progesterone in the second half of your cycle drops back down once your period starts, returning to a baseline that stays relatively stable through the first half of your next cycle.
What the Uterus Is Doing After Shedding
Even while you’re still bleeding, your body is already rebuilding. As soon as progesterone and estrogen bottom out, your brain’s pituitary gland ramps up production of follicle-stimulating hormone, which signals your ovaries to start maturing a new egg. Rising estrogen from the developing follicle then begins stimulating the uterine lining to grow again. By the time your period ends, a fresh layer of tissue is already forming. The entire lining regenerates from scratch each cycle, making the uterus one of the few organs in the body that routinely rebuilds its own tissue.
Heavy vs. Normal Flow
It can be hard to gauge how much you’re actually losing. Research using menstrual cups to measure volume directly found that half of all cycles produced about 81 mL or less, while the 95th percentile reached around 162 mL. Volumes above roughly 170 mL are considered abnormally heavy.
In practical terms, needing to change a pad or tampon every one to two hours for multiple hours, regularly passing large clots, bleeding that lasts longer than eight days, or bleeding that disrupts your ability to go about your daily routine are all signs that your flow is heavier than typical. Heavy periods are common and treatable, but they’re worth bringing up because they can lead to iron deficiency over time.