Period cramps happen because your uterus physically contracts to shed its lining, and the chemicals driving those contractions also cut off oxygen to the surrounding tissue. About 71% of people who menstruate experience cramps, and for roughly 29% of them, the pain is severe. Understanding the chain of events behind that pain helps explain why it varies so much from person to person and cycle to cycle.
The Hormonal Trigger
Every menstrual cycle follows the same basic script. After ovulation, progesterone rises to maintain the uterine lining in case of pregnancy. When pregnancy doesn’t happen, progesterone drops sharply. That drop is the starting gun for everything that follows.
As progesterone falls, the cells lining your uterus respond in two waves. First, they ramp up production of inflammatory chemicals called prostaglandins. Second, immune cells flood into the uterine lining and begin breaking down the tissue so it can be shed. Progesterone normally keeps inflammation in check, so once it withdraws, the floodgates open. The more prostaglandins your body produces during this window, the stronger your cramps will be.
What Prostaglandins Do to Your Uterus
Prostaglandins are the direct cause of the pain. One type in particular triggers the muscle wall of the uterus to contract by releasing a surge of calcium inside muscle cells. These aren’t gentle squeezes. The contractions are strong enough to compress the blood vessels running through the uterine wall, temporarily starving the tissue of oxygen. That oxygen deprivation produces waste products that activate pain-sensing nerves in the lining, the same type of nerves (called C fibers) that detect deep, aching pain elsewhere in the body.
So the pain you feel during cramps is really two things layered on top of each other: the mechanical squeeze of the muscle itself, and the chemical sting of oxygen-starved tissue. It’s a similar mechanism to what happens during a heart attack on a much smaller scale, which is why severe cramps can feel genuinely intense.
Why Cramps Cause Nausea and Diarrhea
Prostaglandins don’t stay neatly contained in the uterus. They circulate through nearby tissue and your bloodstream, and they affect smooth muscle throughout your body. Your gastrointestinal tract is lined with the same type of smooth muscle, so excess prostaglandins can speed up or disrupt your digestion. That’s why many people experience loose stools, diarrhea, nausea, or bloating alongside their cramps. It’s not a coincidence or “just stress.” It’s the same chemical acting on a different organ.
Primary Cramps vs. Secondary Cramps
Most period pain falls into one of two categories, and the distinction matters because the causes and treatments are different.
Primary cramps are the most common type. They happen without any underlying disease. The pain is purely a result of normal prostaglandin activity. Primary cramps typically start six to 12 months after a person’s first period, peak in the late teens and early twenties, and often improve with age. Pain usually begins right when bleeding starts and lasts anywhere from 8 to 72 hours.
Secondary cramps are caused by a structural or medical condition in the pelvis. The most common culprit is endometriosis, where tissue similar to the uterine lining grows outside the uterus. Another frequent cause is adenomyosis, where that same type of tissue burrows into the muscular wall of the uterus itself, causing it to thicken and enlarge to sometimes double or triple its normal size. Fibroids and pelvic infections can also be responsible.
A few patterns suggest cramps may be secondary rather than primary: pain that gets progressively worse over time, cramps that start later in life after years of relatively painless periods, heavy bleeding with large clots, pain during sex, or bleeding between periods. About 35% of people with period pain have a secondary cause contributing to it.
Why Some People’s Cramps Are Worse
The intensity of cramps varies enormously, and that’s largely down to prostaglandin levels. Studies consistently find that people with severe cramps produce significantly more prostaglandins in their uterine lining than people with mild or no cramps. Your body’s individual inflammatory response is partly genetic, which is why painful periods often run in families.
Other factors that can amplify cramps include having a heavier flow (more lining to shed means more prostaglandin release), starting your period at a younger age, smoking, and high levels of psychological stress. Being physically active, on the other hand, tends to correlate with less severe pain, likely because exercise improves pelvic blood flow and releases natural pain-relieving chemicals.
How Pain Relievers Target the Problem
Anti-inflammatory pain relievers like ibuprofen and naproxen work directly on the root cause. They block the enzymes that produce prostaglandins, reducing both the intensity of uterine contractions and the downstream oxygen deprivation that triggers pain nerves. This is why they tend to work better for period cramps than acetaminophen (Tylenol), which reduces pain signals in the brain but doesn’t lower prostaglandin levels in the uterus.
Timing matters. These medications work best when taken at the very first sign of cramps or even just before bleeding starts, because they prevent prostaglandin buildup rather than trying to counteract prostaglandins already circulating. If you wait until the pain is severe, there’s a larger chemical load to overcome.
Hormonal birth control is another common approach. By suppressing ovulation and thinning the uterine lining, it reduces the amount of tissue that needs to be shed and the volume of prostaglandins produced in the process. For some people, this eliminates cramps almost entirely.
Why Heat Actually Works
Placing a heating pad on your lower abdomen isn’t just comforting. Heat at around 40 to 45°C (104 to 113°F) penetrates about a centimeter into tissue, enough to relax the abdominal muscles, improve blood circulation in the pelvis, and reduce the nerve compression caused by swelling and congestion. By restoring blood flow, heat directly counteracts the ischemia (oxygen deprivation) that’s generating pain signals. Clinical trials have found heat therapy comparable to over-the-counter pain relievers for mild to moderate cramps, and combining the two tends to work better than either alone.
Signs Your Cramps Need a Closer Look
Pain that doesn’t respond to standard anti-inflammatory medication is worth investigating, because it can point to an underlying condition like endometriosis or adenomyosis. The same is true if your cramps are severe enough to regularly keep you home from work or school, if the pain feels noticeably worse than it used to, or if you develop new symptoms like heavy bleeding, pain outside your period, or pain during sex. Period pain caused by a medical condition tends to get worse over time rather than staying stable, so a changing pattern is particularly worth paying attention to.