Period pain is caused by natural chemicals called prostaglandins that force the uterus to contract, squeezing blood vessels and temporarily cutting off oxygen to the muscle tissue. About 71% of menstruating people worldwide experience this pain to some degree. For most, the process is a normal part of the menstrual cycle. For others, an underlying condition like endometriosis or fibroids is amplifying the pain beyond what prostaglandins alone would cause.
How Prostaglandins Trigger Cramps
The pain starts with a hormonal shift. Throughout the second half of your cycle, progesterone keeps prostaglandin levels in check by boosting an enzyme that breaks them down and by suppressing the inflammatory pathways that produce them. When progesterone drops sharply just before your period, that suppression lifts. Two things happen at once: the enzyme that was clearing prostaglandins declines, and the enzyme responsible for making them ramps up. The result is a surge of prostaglandins concentrated in the lining of the uterus.
These prostaglandins cause the uterine muscle to contract intensely and in a disorganized pattern. The contractions compress blood vessels running through the muscle wall, reducing blood flow (a state called ischemia). Without adequate blood flow, oxygen levels drop and the tissue shifts to a less efficient form of energy production that generates painful byproducts. This is essentially the same mechanism behind a muscle cramp in your calf, just happening inside the uterus.
People with painful periods don’t just feel more pain. They actually produce more prostaglandins. Studies measuring prostaglandin levels in menstrual fluid consistently find higher concentrations in people with painful periods compared to pain-free controls, with some studies finding two to ten times more. Blood levels of one key prostaglandin run roughly 1.7 to 2.7 times higher in people with cramps. Another hormone, vasopressin, also plays a role by further constricting blood vessels and increasing contractions.
Primary Period Pain: No Underlying Disease
When cramps are caused purely by this prostaglandin-driven process, with no other condition contributing, it’s called primary dysmenorrhea. This is by far the most common type, affecting an estimated 73% of those who experience period pain. It typically begins within the first two years of getting your period, once cycles become regular and ovulation is established.
The pain pattern is recognizable: cramping starts within a few hours of bleeding and usually resolves within 72 hours. It centers in the lower abdomen and can radiate to the lower back or inner thighs. It tends to be worst on the first day or two, then fades as prostaglandin levels drop. Some people also experience nausea, diarrhea, or headaches because excess prostaglandins can enter the bloodstream and affect other tissues.
When Another Condition Is Driving the Pain
Secondary dysmenorrhea means something beyond normal prostaglandin activity is involved. This type can start at any age but is more likely to appear in your 30s or 40s as a new or worsening symptom. The pain often lasts longer than three days, may begin before bleeding starts, or may be present outside of your period altogether. Several conditions can be responsible.
Endometriosis
Endometriosis occurs when tissue similar to the uterine lining grows in places it shouldn’t, such as the pelvic cavity, ovaries, or bowel surface. This tissue responds to hormonal changes just like the lining inside the uterus, building up and breaking down each cycle. But because it has no way to exit the body, it triggers chronic local inflammation. The immune system sends inflammatory cells to the area, which release chemicals that promote further tissue growth, scarring, and adhesions. Over time, this creates a self-reinforcing cycle: the misplaced tissue produces its own estrogen, which drives more prostaglandin production, which drives more estrogen, steadily worsening pain. Scar tissue (fibrosis) can also form, binding organs together and causing pain with movement, bowel changes, or sex.
Adenomyosis
Adenomyosis is a related condition where endometrial tissue grows into the muscular wall of the uterus itself. This causes the uterus to enlarge and makes contractions during your period more painful. It often produces heavy bleeding alongside severe cramps and is more common in people who have had children or uterine surgery.
Fibroids
Uterine fibroids are noncancerous growths in or on the uterine wall. They can cause excessive or painful bleeding during your period, along with a feeling of fullness or pressure in the lower abdomen. Larger fibroids can press on nearby structures, leading to back pain, frequent urination, or constipation. Not all fibroids cause symptoms, but those that distort the uterine cavity or grow large enough to affect blood flow tend to make periods significantly more painful.
Other Causes
Pelvic inflammatory disease (an infection of the reproductive organs, often from untreated sexually transmitted infections) can cause ongoing pelvic pain that worsens during periods. Endometrial polyps, which are small growths on the uterine lining, can contribute to heavier and more painful bleeding. In rarer cases, a narrow cervical opening can slow the flow of menstrual blood, increasing pressure inside the uterus.
Risk Factors That Affect Severity
Family history is the strongest known predictor. If your mother or sister has painful periods, your risk is substantially higher. Studies have found that having a family history of dysmenorrhea increases the odds by anywhere from 4 to 20 times, suggesting a significant genetic component in how much prostaglandin your body produces or how your uterus responds to it.
Age matters too. Primary period pain is most common in teens and young adults, and for many people it gradually improves through the late 20s and after childbirth. Heavier or longer periods are consistently linked to worse pain, likely because more uterine lining means more prostaglandin production. The relationship between smoking, early menarche, and period pain has been studied repeatedly, but the results are mixed enough that researchers can’t draw firm conclusions on those factors.
What Actually Helps
Because prostaglandins are the direct cause of primary period pain, anti-inflammatory pain relievers (like ibuprofen or naproxen) work by blocking the enzyme that produces them. They’re most effective when taken at the first sign of pain or even just before your period starts, before prostaglandin levels peak. Waiting until pain is severe means prostaglandins have already been released and are harder to counteract.
Heat therapy is a genuinely effective alternative. A large meta-analysis of 22 randomized trials found that heat and anti-inflammatory medications provide comparable pain relief, with only about a 4% difference in effectiveness over three months. The practical advantage of heat is safety: people using heat therapy experienced roughly 70% fewer side effects than those taking anti-inflammatory drugs. A heating pad, hot water bottle, or adhesive heat wrap applied to the lower abdomen can relax uterine muscle and improve blood flow to oxygen-starved tissue.
Hormonal birth control reduces period pain by thinning the uterine lining, which means fewer prostaglandins are produced in the first place. For people with secondary causes like endometriosis, treatment depends on the underlying condition and can range from hormonal management to surgical removal of problematic tissue.
Signs Your Pain May Need Investigation
Most period pain is unpleasant but manageable. Certain patterns, though, suggest something beyond normal prostaglandin activity. Pain that doesn’t respond to anti-inflammatory medication and interferes with daily life warrants a closer look. The same goes for cramps that suddenly become much worse than your usual pattern, severe cramps appearing for the first time after age 25, pain that occurs outside of your period, or fever accompanying menstrual pain. These patterns don’t guarantee a serious problem, but they’re the situations where imaging or further evaluation can identify treatable conditions like endometriosis, fibroids, or infection.