Period cramps happen because your uterus physically contracts to shed its lining, and those contractions temporarily cut off their own blood supply, starving the muscle of oxygen. It’s the same basic mechanism that causes a heart attack to hurt, just on a smaller and shorter scale. About 60% of people with a uterus experience at least mild cramping each cycle, and 5% to 15% report pain severe enough to disrupt daily life.
How Prostaglandins Drive the Pain
The process starts with chemicals called prostaglandins, which your uterine lining produces in rising amounts as your period approaches. One type in particular acts as a powerful trigger for muscle contraction. It also constricts blood vessels running through the uterus and its lining. So you end up with a double hit: the muscle squeezes harder, and the blood vessels feeding it narrow at the same time.
As the uterine wall contracts, it physically compresses the blood vessels that run through it. Research using imaging in primates and humans shows that at the peak of an average-intensity contraction, blood flow through the uterus can stop entirely. That temporary loss of oxygen is what generates the pain. Think of it like a charley horse in your calf: the muscle clenches so hard it chokes off its own circulation, and the oxygen-deprived tissue sends urgent pain signals. Once the contraction relaxes, blood returns, oxygen is restored, and the pain eases until the next wave.
People who have more painful periods consistently show higher prostaglandin levels in their menstrual fluid. The more prostaglandins your lining produces, the stronger and more frequent the contractions, the more blood flow gets interrupted, and the worse the cramping feels.
Other Chemicals That Make It Worse
Prostaglandins aren’t acting alone. Your body also produces leukotrienes, compounds that appear to make pain-sensing nerve fibers in the uterus more sensitive. Women whose cramps don’t respond to standard anti-inflammatory painkillers tend to have substantially higher leukotriene levels in their uterine lining, which may explain why those treatments fall short for some people.
Vasopressin, a hormone released by the brain, also plays a role. It can increase the uterus’s sensitivity to contractions, further reduce blood flow, and may even stimulate more prostaglandin production. Vasopressin levels tend to be elevated during menstruation, adding another layer to the pain cycle.
When Cramps Typically Hit
Pain usually begins one to three days before bleeding starts, peaks about 24 hours after your period begins, and fades within two to three days. That timeline maps directly onto prostaglandin levels: production ramps up as your hormone levels drop before your period, peaks early in menstruation, and falls off as the lining finishes shedding. For most people, the worst of it is concentrated in the first 48 hours of bleeding.
Why Some People Get Worse Cramps
Cramp severity varies widely, and several factors influence where you fall on that spectrum. Age matters: cramps tend to be most intense during adolescence and often improve with age, particularly after pregnancy. Body weight plays a role too. A large study of nearly 10,000 women found that those with a BMI under 18.5 had a 34% higher risk of significant cramping, while those with a BMI of 30 or above had a 22% higher risk, compared to women in the middle range. Smoking is another established risk factor, and quitting has been shown to help.
Genetics likely influence how much prostaglandin your uterine lining produces, though the exact genes involved aren’t well mapped. If your mother or sister had severe cramps, your odds of experiencing them go up.
Why Anti-Inflammatory Painkillers Work
Over-the-counter anti-inflammatory painkillers like ibuprofen work by blocking the enzyme your body uses to manufacture prostaglandins. Less prostaglandin production means weaker contractions, better blood flow, and less oxygen deprivation in the uterine muscle. The key is timing: taking them before the pain peaks, ideally at the first sign of cramping or even just before your period starts, gives the drug time to lower prostaglandin levels before they build up. Waiting until you’re already in significant pain means a backlog of prostaglandins is already doing its work, and the medication has to play catch-up.
Hormonal birth control takes a different approach. By thinning the uterine lining or preventing ovulation, it reduces the amount of tissue available to produce prostaglandins in the first place. That’s why people on hormonal contraceptives often notice lighter periods and milder cramps.
Normal Cramps vs. an Underlying Condition
Standard period cramps with no underlying cause are called primary dysmenorrhea. The pain follows a predictable pattern: it shows up around the start of your period and resolves within a few days. When cramps are caused by a condition affecting the reproductive organs, such as endometriosis, fibroids, or pelvic infections, that’s secondary dysmenorrhea, and the pattern looks different.
Secondary dysmenorrhea typically starts earlier in the cycle, sometimes days before any bleeding, and lasts longer, often persisting until bleeding stops completely. The pain may also worsen over time rather than staying stable from cycle to cycle. If your cramps have changed significantly, started later in life after years of relatively painless periods, or don’t respond at all to anti-inflammatory painkillers, that shift in pattern is worth investigating.