Menstrual cramps are caused by natural hormone-like compounds called prostaglandins that force the uterine muscle to contract, squeezing out its lining each month. About 71% of menstruating people worldwide experience these cramps, and while the basic process is normal, the severity varies widely depending on prostaglandin levels, underlying health conditions, and individual risk factors.
How Prostaglandins Trigger the Pain
The story starts at the end of your cycle. If pregnancy doesn’t occur, the small hormone-producing structure left behind by your ovary (the corpus luteum) breaks down, and your progesterone levels drop sharply. Progesterone normally keeps inflammation in check by suppressing certain immune pathways in the uterine lining. When it drops, that suppression lifts, and the lining floods with inflammatory signals and prostaglandins.
Prostaglandins, especially one called PGF2α, are the direct trigger for cramping. They bind to receptors on the muscle cells of the uterus and set off a chain reaction that floods those cells with calcium. That calcium surge is what makes the muscle contract. The contractions serve a purpose: they help shed the uterine lining and push menstrual blood out through the cervix. But they also compress the small blood vessels feeding the uterus, temporarily cutting off oxygen to the tissue. That combination of strong contractions and reduced blood flow is what you feel as pain.
People with more severe cramps tend to have measurably higher prostaglandin levels in their menstrual fluid. This is why anti-inflammatory pain relievers work well for cramps: they block the enzyme that produces prostaglandins in the first place, reducing both the contractions and the oxygen deprivation.
Primary Cramps vs. Secondary Cramps
Doctors divide menstrual cramps into two categories, and the distinction matters because the causes and treatments are different.
Primary dysmenorrhea is the common cramping that comes before or during your period with no underlying disease. It typically starts within the first year or two of getting periods and follows a predictable pattern: pain begins as bleeding starts (or just before), peaks within the first 24 to 48 hours, and fades as the period winds down. This type is purely driven by prostaglandin production and accounts for the majority of menstrual pain.
Secondary dysmenorrhea is cramping caused by a structural or medical problem in the reproductive organs. It affects roughly 35% of people with painful periods. The key differences are that pain tends to get worse over time, often starts days before bleeding begins, and may not go away after the period ends. If your cramps have changed significantly from what they used to be, or if they’ve become progressively more severe over months or years, a secondary cause is worth investigating.
Conditions That Cause Secondary Cramps
Endometriosis
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus, on the ovaries, fallopian tubes, or pelvic surfaces. These growths respond to the same hormonal cycle as the uterine lining, building up and breaking down each month. But because the tissue has nowhere to drain, it triggers chronic inflammation, irritates surrounding nerves, and can form adhesions that pull on pelvic structures. The pain often extends beyond periods, showing up during ovulation, sex, or bowel movements.
Adenomyosis
In adenomyosis, the uterine lining invades the muscular wall of the uterus itself. This displaced tissue still produces prostaglandins and responds to hormonal changes, which triggers contractions from within the muscle wall. The combination of inflammation, excess prostaglandin production, and abnormal uterine contractions makes periods heavier and significantly more painful. Adenomyosis is most common in people in their 30s and 40s, and it often coexists with endometriosis.
Fibroids
Uterine fibroids are noncancerous growths in or on the uterine wall. They can distort the shape of the uterus, increase its surface area, and interfere with its ability to contract efficiently. Not all fibroids cause pain, but those that grow into the uterine cavity or press on surrounding tissue can make cramps noticeably worse and periods heavier.
Pelvic Inflammatory Disease
Pelvic inflammatory disease (PID) is an infection of the reproductive organs, usually caused by sexually transmitted bacteria. Even after the infection is treated, it can leave behind scar tissue and pockets of inflammation in the fallopian tubes and surrounding structures. That scarring can cause chronic pelvic pain that flares during periods, ovulation, or sex, sometimes lasting months or years after the original infection has cleared.
Cervical Stenosis
A cervical opening that is unusually narrow can partially block the flow of menstrual blood. When the uterus contracts to push blood through a restricted channel, pressure builds inside the uterus, intensifying pain. In severe cases, blood can accumulate in the uterus entirely, causing significant cramping and pelvic pressure.
Risk Factors That Make Cramps Worse
Several factors influence how severe your cramps are likely to be, some modifiable and some not.
- Age at first period: Starting your period before age 12 is associated with a 37% higher risk of significant menstrual pain compared to those who start later. Earlier hormonal exposure may prime the uterus for stronger prostaglandin responses.
- Smoking: Nicotine constricts blood vessels, reducing blood flow to the uterus and increasing prostaglandin production. This essentially amplifies the same oxygen-deprivation mechanism that already drives cramp pain.
- Body weight: Research from a large Taiwanese study found that people with a BMI under 25 had a modestly lower risk of painful periods compared to those with higher BMIs, though the relationship is complex and influenced by other hormonal factors.
- Heavy periods: More menstrual fluid generally means more prostaglandins, which means stronger contractions. Conditions that increase flow, like fibroids or adenomyosis, compound the problem.
- Family history: If your mother or sisters have severe cramps, you’re more likely to as well. Prostaglandin production levels and uterine sensitivity appear to run in families.
What’s Happening in Your Body During a Cramp
A menstrual cramp is functionally similar to a labor contraction, just smaller in scale. The uterine muscle tightens, compresses blood vessels, and temporarily starves tissue of oxygen. Pain signals travel through two different nerve pathways in the pelvis, both sympathetic and parasympathetic nerves in the lower spine. This dual-pathway signaling is part of why menstrual pain can feel so diffuse, radiating into the lower back, thighs, and even the bowels rather than staying neatly located in the uterus.
The intensity of each cramp depends on how forcefully the muscle contracts and how long the blood supply is interrupted. In people with high prostaglandin levels, the uterus can generate pressures comparable to those during active labor. That’s not an exaggeration or a metaphor. It’s a measured physiological finding that helps explain why some people are genuinely incapacitated by their periods while others barely notice them.
Why Cramps Change Over Time
Your experience with menstrual cramps isn’t fixed. Primary cramps often peak in the late teens and early 20s and gradually improve with age, particularly after pregnancy, which can alter nerve pathways and blood supply to the uterus. Hormonal contraceptives also reduce cramping for many people by thinning the uterine lining and lowering prostaglandin production.
Cramps that are getting worse over time tell a different story. Progressively worsening pain, especially if it’s expanding beyond the first day or two of your period, often signals a secondary cause like endometriosis or adenomyosis developing. These conditions tend to be progressive, meaning the pain builds gradually over months or years rather than appearing all at once. The shift is easy to dismiss as “just bad periods,” but the pattern itself is diagnostically meaningful.