Period cramps are caused by your uterus contracting to shed its lining, driven by hormone-like chemicals called prostaglandins. Up to 91% of people who menstruate experience cramps, and about 60% of adolescents report moderate to severe pain. The process is a normal part of menstruation, but the intensity varies widely from person to person depending on how much prostaglandin your body produces.
What Triggers the Cramping
In the second half of your menstrual cycle, progesterone keeps the uterine lining thick and stable. When you don’t become pregnant, progesterone levels drop sharply. That drop is the starting signal. Without progesterone holding things together, the cells lining the uterus begin breaking down and releasing prostaglandins, particularly one called prostaglandin F2-alpha.
Prostaglandins are not technically hormones. They’re chemical messengers produced locally in the uterine tissue, and their job is to make the muscular wall of the uterus contract. Those contractions squeeze the lining away from the uterine wall so it can be shed as your period. The more prostaglandins your body releases, the stronger and more frequent the contractions, and the more painful the cramps.
How Prostaglandins Cause Pain
Prostaglandin F2-alpha works by binding to a specific receptor on uterine muscle cells. When it locks onto that receptor, it triggers a chain reaction that floods the muscle cells with calcium. Calcium is what makes any muscle contract, whether it’s your bicep or your uterus. The prostaglandin essentially tells the muscle to squeeze by opening the floodgates for calcium release inside the cell.
This creates two sources of pain. First, the contractions themselves. The uterus is a powerful muscle, and during intense cramps, it can generate pressure comparable to what happens during labor, just for shorter bursts. Second, when the muscle contracts hard enough, it temporarily compresses the small blood vessels that supply the uterine wall. This briefly cuts off oxygen to the tissue, producing a type of pain similar to what you feel when a muscle cramps during exercise. That oxygen deprivation is part of why cramps can feel like a deep, squeezing ache rather than a sharp surface pain.
Prostaglandins also trigger inflammation in the surrounding tissue, which amplifies pain signals. Some prostaglandins enter the bloodstream, which is why cramps often come with side effects that seem unrelated to the uterus: nausea, diarrhea, headaches, and general fatigue. Prostaglandins affect smooth muscle throughout the body, so if enough of them circulate, they can cause intestinal cramping and loose stools too.
When Cramps Start and How Long They Last
Cramps typically begin just before or at the very start of bleeding, when prostaglandin levels peak. The first one to two days of your period are usually the worst. Pain tends to be most intense during the heaviest flow, then gradually fades as prostaglandin levels decline and the bulk of the lining has been shed. Most people find that cramps are gone or minimal by day three or four.
The sensation is often described as a dull, throbbing ache in the lower abdomen, sometimes radiating to the lower back and inner thighs. It can come in waves as the uterus contracts and relaxes rhythmically, or feel like a constant low-grade pressure with periodic spikes of sharper pain.
Primary vs. Secondary Cramps
There are two distinct categories of period pain, and the distinction matters because the causes and treatments are different.
Primary dysmenorrhea is the common, garden-variety cramping described above. It’s caused purely by prostaglandins, with no underlying disease. It typically starts six to 12 months after a person’s first period, once ovulatory cycles become regular. Pain tends to peak in the late teens and early twenties and often becomes less severe after childbirth or with age.
Secondary dysmenorrhea is period pain caused by an underlying condition, most commonly endometriosis (where tissue similar to the uterine lining grows outside the uterus), adenomyosis (where that tissue grows into the muscular wall of the uterus), or fibroids. This type of pain often starts later in life or gets progressively worse over time. The cramps may begin earlier in the cycle, last longer than the period itself, or be accompanied by pain during sex or heavy irregular bleeding. If you’ve always had manageable periods and the pain suddenly becomes severe, or if you develop significant cramps for the first time after age 25, that pattern points toward a secondary cause worth investigating.
Why Some People Have Worse Cramps
The single biggest factor is prostaglandin levels. Studies consistently show that people with severe cramps produce significantly more prostaglandins in their uterine lining than people with mild or no cramps. This is largely genetic, which is why severe period pain often runs in families.
Other factors that can worsen cramps include having a heavier flow (more lining to shed means more prostaglandin production), being younger (the cervical opening is narrower in adolescence, creating more pressure), smoking, and high levels of stress. People who started menstruating before age 12 and those with longer or irregular cycles also tend to report more pain.
How Pain Relief Targets the Cause
Anti-inflammatory painkillers like ibuprofen and naproxen don’t just mask period pain. They work by blocking the enzyme (cyclooxygenase) that your body uses to manufacture prostaglandins in the first place. Less prostaglandin production means weaker contractions, less inflammation, and less pain. This is why these medications work best when taken at the very first sign of cramps or even slightly before your period starts. If you wait until the pain is severe, prostaglandins have already been released and are already acting on the muscle cells.
Hormonal birth control takes a different approach. By preventing ovulation or thinning the uterine lining, it reduces the amount of tissue that needs to be shed each cycle. Less lining means less prostaglandin production at the source. This is why many people on hormonal contraceptives notice their cramps become significantly lighter or disappear entirely.
Heat also has a real physiological effect. Applying a heating pad to the lower abdomen increases blood flow to the uterus, which helps counteract the oxygen deprivation caused by strong contractions. Clinical trials have found that continuous topical heat can be as effective as ibuprofen for mild to moderate cramps. Exercise works through a similar mechanism: increased circulation plus the release of your body’s natural pain-dampening chemicals.
Signs That Cramps Are Not Typical
Some level of cramping is a normal part of menstruation, but certain patterns suggest something beyond routine prostaglandin activity. Pain that disrupts your daily life every single month, pain that has gotten progressively worse over several cycles, and the new onset of severe cramps after age 25 are all signals worth bringing to a healthcare provider. Other red flags include cramps that don’t respond at all to anti-inflammatory medication, pain that persists well after your period ends, and unusually heavy bleeding that soaks through a pad or tampon in less than an hour.
In adolescence, severe period pain is linked to school absences, reduced concentration, and avoidance of sports and social activities. Research published in The Lancet has also found an association between moderate to severe cramps in adolescence and chronic pain conditions later in adulthood, suggesting that early and effective management of period pain is more than just a comfort issue.