Period cramps happen because your uterus physically contracts to shed its lining, and the chemical that drives those contractions also triggers pain and inflammation. Up to 91% of people who menstruate experience some degree of cramping, with severe pain affecting roughly 2% to 29% depending on the population studied. The process is straightforward biology, but understanding exactly what’s happening in your body helps explain why cramps can range from barely noticeable to debilitating.
The Role of Prostaglandins
The main culprit behind period pain is a chemical called prostaglandin F2α. Your uterine lining produces it in increasing amounts as your period approaches. When menstruation begins, prostaglandin levels are at their highest. This chemical binds to receptors on the muscle cells of your uterus and triggers a flood of calcium inside those cells, which is the direct signal that makes the muscle contract. More prostaglandins mean stronger, more frequent contractions, and more pain.
Prostaglandins don’t just cause contractions. They also activate inflammatory pathways in the uterine muscle, ramping up the production of additional inflammatory molecules. This creates a feedback loop: the initial wave of prostaglandins causes your uterus to produce even more of the enzymes that generate prostaglandins, intensifying both the contractions and the inflammation around them. That’s why the first day or two of your period tends to be the worst. As bleeding continues and the uterine lining sheds, prostaglandin levels drop, and the pain eases.
Why It Actually Hurts
The pain isn’t just from the squeezing itself. Prostaglandin F2α is also a powerful vasoconstrictor, meaning it narrows the blood vessels feeding the uterus. When the uterine muscle contracts hard while its blood supply is reduced, the tissue temporarily doesn’t get enough oxygen. This is essentially the same mechanism behind the pain of a heart attack or a muscle cramp in your leg: ischemia, or oxygen starvation in the tissue. Women with more severe cramps tend to have measurably higher intrauterine pressure and more restricted blood flow to the uterus than those with milder symptoms.
Typical menstrual cramps feel like a cramping or aching pain in the lower abdomen, starting right around when bleeding begins and lasting anywhere from 8 to 72 hours.
Why Cramps Cause Nausea and Diarrhea
If your period also comes with nausea, diarrhea, or stomach cramps, prostaglandins are responsible for that too. These chemicals are produced locally in the uterus, but some escape into the bloodstream before the body can break them down. Once circulating, they stimulate both the muscle contractions and secretory activity of the gut. Your intestines essentially respond to the same chemical signal your uterus is responding to, which is why your digestive system can feel so disrupted during your period.
Who Gets Worse Cramps
Several factors are associated with more severe period pain. Starting your period before age 12 is one. Having long, heavy, or irregular cycles increases risk, as does having a low body mass index (under 20). Exposure to tobacco smoke, whether firsthand or secondhand, is linked to worse cramps. The presence of premenstrual symptoms like bloating and mood changes also tends to predict more painful periods.
Cramps typically begin within a year or two of a person’s first period and, for many people, gradually improve with age or after pregnancy. But the severity varies enormously. About 7% to 15% of women report pain severe enough to limit daily activities. Among adolescents and young adults, that number jumps to around 41%.
How Anti-Inflammatory Painkillers Help
Common over-the-counter painkillers like ibuprofen and naproxen work directly on the root cause of period cramps, not just the pain. They block the enzyme (cyclooxygenase) that your body uses to produce prostaglandins in the first place. Fewer prostaglandins means less intense contractions, less blood vessel constriction, less tissue oxygen deprivation, and less inflammation. This is why these medications tend to work better for period cramps than acetaminophen (Tylenol), which reduces pain but doesn’t lower prostaglandin production. Taking them just before or at the very start of your period, before prostaglandin levels peak, tends to be more effective than waiting until pain is already severe.
Heat applied to the lower abdomen also helps by increasing blood flow to the uterus, counteracting some of the ischemia that prostaglandins cause.
When Pain Signals Something Else
The cramping described above is called primary dysmenorrhea. It’s caused purely by the normal biochemistry of menstruation, not by any underlying disease. But period pain can also be caused by conditions like endometriosis (where tissue similar to the uterine lining grows outside the uterus), ovarian cysts, or fibroids. This is called secondary dysmenorrhea.
A useful rule of thumb from the Mayo Clinic: normal menstrual cramping should be tolerable and should not require you to miss school, work, or normal activities. Pain that goes beyond that, that extends well outside your actual period, or that gets progressively worse over time rather than following the same predictable pattern each month may point to an underlying condition. Endometriosis in particular can mimic other problems like irritable bowel syndrome, since it can cause diarrhea, constipation, and abdominal cramping that overlap with digestive issues. If your pain doesn’t respond to over-the-counter anti-inflammatories or is disrupting your life, a pelvic exam and imaging can help identify or rule out these conditions.