Period cramps happen because your uterus contracts to shed its lining, and those contractions temporarily squeeze off their own blood supply, starving the muscle of oxygen. About 71% of people who menstruate experience this pain, making it one of the most common physical experiences in reproductive health. The intensity ranges from a mild ache to pain severe enough to disrupt work, school, and sleep.
What’s Happening Inside Your Uterus
Each menstrual cycle, the lining of your uterus thickens in preparation for a potential pregnancy. When pregnancy doesn’t occur, your progesterone levels drop sharply in the days before your period starts. That hormonal shift triggers the release of chemicals called prostaglandins from the uterine lining.
Prostaglandins do two things simultaneously. They cause the muscular wall of the uterus to contract, which helps push out the shedding lining. But they also constrict the blood vessels feeding that same muscle. The result is a brief period of reduced blood flow, similar to what happens when you clench a fist so tightly your hand turns white. Without adequate blood flow, the muscle tissue is temporarily starved of oxygen, and the oxygen-deprived cells release waste products that activate pain nerves.
The pain nerves involved are slow-conducting fibers that produce a deep, aching, crampy sensation rather than a sharp sting. This is why menstrual cramps tend to feel like a dull, throbbing pressure low in the abdomen rather than a stabbing pain. The contractions come in waves because the uterus squeezes and releases rhythmically, which is why cramps pulse rather than stay constant. Another hormone, vasopressin (released from the brain), can amplify this process by making the uterine muscle more sensitive and further reducing blood flow.
Why Some People Get Worse Cramps
The severity of cramps correlates directly with how much prostaglandin your uterine lining produces. People with more painful periods have measurably higher prostaglandin levels in their menstrual fluid. But several other factors raise the odds of more intense or longer-lasting pain.
- Body weight: People at the highest end of the weight spectrum have roughly 75% higher odds of experiencing menstrual cramps.
- Smoking: Current smokers have about a 50% increase in the odds of pain lasting more than two days, likely because nicotine constricts blood vessels and worsens the oxygen deprivation already happening in the uterus.
- Alcohol during cramps: Drinking while you’re already cramping increases both the severity and duration of the pain.
- Early first period: Starting menstruation at age 11 or younger is linked to more severe and longer-lasting cramps.
- Longer cycles: People with cycles around 35 days are more likely to have cramps than those with 28- to 29-day cycles.
Cramps also tend to be most intense in your teens and twenties, then gradually improve with age. For many people, giving birth reduces cramp severity, possibly because the cervix stretches permanently, allowing menstrual tissue to pass more easily.
When Cramps Signal Something Else
The type of cramps described above, caused purely by prostaglandins and contractions with no underlying disease, is called primary dysmenorrhea. It typically begins within a year or two of your first period and follows a predictable pattern: pain starts just before or at the onset of bleeding and fades within two to three days.
Secondary dysmenorrhea is period pain caused by an identifiable condition in the pelvis. It often shows up later in life, sometimes not appearing until your 30s or 40s, and the pain pattern tends to be different. It may start earlier in your cycle, last longer, or worsen over the years instead of improving. The most common causes include:
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus, attaching to organs like the ovaries, fallopian tubes, or bowel. This tissue still responds to your monthly hormonal shifts, swelling and bleeding with each cycle but with no way to exit the body. The result is inflammation, scarring, and adhesions that pull on surrounding structures. Up to 20% of people with endometriosis develop a severe form where the tissue invades more than 5mm deep into the pelvic lining.
Adenomyosis is a related condition where uterine lining tissue burrows into the muscular wall of the uterus itself. Each cycle, the embedded tissue swells, causing the uterus to enlarge and cramp more intensely. This condition is more common in people who have had children or uterine surgery.
Fibroids are noncancerous growths in the uterine wall. They’re sensitive to hormones, often growing during pregnancy and shrinking after menopause. Depending on their size and location, fibroids can distort the uterine cavity and cause heavier, more painful periods.
Pelvic inflammatory disease is an infection, often from sexually transmitted bacteria, that spreads from the cervix into the uterus and fallopian tubes. It causes inflammation that makes periods more painful and can also produce pain between periods, unusual discharge, or fever.
How Anti-Inflammatory Painkillers Work on Cramps
Since prostaglandins are the primary driver of menstrual pain, the most effective over-the-counter treatment is a class of painkillers that blocks prostaglandin production. Ibuprofen and naproxen both work this way. In a large network analysis comparing common painkillers for period cramps, ibuprofen was roughly 10 times more effective than placebo, and naproxen was about 4 times more effective. Aspirin, by contrast, performed no better than a sugar pill in the same analysis.
Timing matters more than most people realize. These medications work by preventing prostaglandin production, not by neutralizing prostaglandins that already exist. Taking ibuprofen or naproxen at the very first sign of cramps, or even the day before you expect your period to start, gives the medication time to reduce prostaglandin levels before contractions peak. Waiting until the pain is already severe means the inflammatory cascade is well underway, and the medication has to play catch-up.
Hormonal birth control is another common approach. By thinning the uterine lining, these methods reduce the total amount of prostaglandin-producing tissue. Less lining means fewer prostaglandins, weaker contractions, and less pain. This is why people on hormonal contraceptives often notice their cramps improve significantly.
What Severe Cramps Can Look Like
Normal period cramps concentrate in the lower abdomen but can radiate to the lower back and inner thighs. They’re usually worst during the first one to two days of bleeding and respond at least partially to painkillers or a heating pad.
Pain that doesn’t fit this pattern is worth paying attention to. Cramps that get progressively worse year after year, pain during sex, pain with bowel movements or urination during your period, or bleeding that soaks through a pad or tampon every hour can all point toward conditions like endometriosis or adenomyosis. Pain that shows up outside your period window, especially if accompanied by fever or unusual discharge, suggests infection rather than normal menstrual cramping. And if over-the-counter painkillers barely touch the pain or you’re regularly missing work or school because of your period, that level of disruption isn’t something you need to push through.