Endometriosis affects millions of women, often causing years of pelvic pain, heavy periods and fertility problems. A large nationwide study now adds another concern to the picture: women diagnosed with endometriosis or adenomyosis had higher subsequent risks of certain cancers, with some of the strongest associations involving specific types of ovarian cancer.
The findings do not mean that endometriosis usually leads to cancer. Ovarian cancer remains uncommon even among women with endometriosis, and the absolute lifetime risk is still low.
What the study does show is that certain ovarian cancer subtypes appeared considerably more often among women with endometriosis or adenomyosis than among women without those diagnoses.
Researchers followed more than 49,000 women
The nationwide Swedish study included 49,133 women who were diagnosed with endometriosis or adenomyosis between 1997 and 2018. Their cancer diagnoses were compared with those occurring among women without either condition.
The full study drew on national patient and cancer registries, giving researchers nearly 994,000 person-years of follow-up. The average follow-up period among women with endometriosis or adenomyosis was just over 20 years.
During the study period, 1,784 women in the endometriosis or adenomyosis group were diagnosed with a gynecological or breast cancer. Some cancers were diagnosed during the same calendar year as the underlying condition, while others appeared years later.
That timing became important because cancer risks looked very different once researchers separated cancers detected around the time of the endometriosis diagnosis from those that developed later.
Takeaway: Following tens of thousands of women for roughly two decades allowed researchers to examine cancer risk well beyond the initial endometriosis diagnosis.
Ovarian cancer stood out years later
When researchers excluded cancers diagnosed during the same calendar year as endometriosis or adenomyosis, the subsequent risk of epithelial ovarian cancer remained about 1.9 times as high as expected.
The distinction matters because cancers discovered at the same time as endometriosis can complicate interpretation. A woman may undergo testing or surgery because doctors suspect cancer and have endometriosis discovered during that investigation, or the reverse may happen.
Looking only at later years reduces some of that immediate detection effect.
The increased ovarian cancer association also persisted beyond 10 years of follow-up, suggesting it could not be explained entirely by intense medical investigation around the time endometriosis was first diagnosed.
Researchers did not find the same long-term pattern for every gynecological cancer. In subsequent years, cervical and endometrial cancer rates were actually lower than expected in the group.
Takeaway: The longer-term association was especially notable for epithelial ovarian cancer rather than representing a broad increase across every gynecological cancer.
Clear cell ovarian cancer showed the strongest association
The biggest difference appeared when researchers stopped treating ovarian cancer as a single disease and examined individual subtypes. Women with endometriosis or adenomyosis had about 7.7 times the expected rate of subsequent ovarian clear cell cancer.
Endometrioid ovarian cancer was also elevated, at about 2.8 times the expected rate, while low-grade serous ovarian cancer was associated with a smaller increase of roughly 1.5 times.
These results fit with earlier research suggesting that endometriosis has a particularly strong relationship with clear cell and endometrioid ovarian cancers.
Researchers are still investigating the biological explanation. Endometriosis involves chronic inflammation, and scientists have also detected some cancer-related genetic mutations in both endometriosis lesions and certain ovarian tumors.
That does not mean an endometriosis lesion will necessarily become cancerous. It suggests there may be shared biological pathways that help explain why particular ovarian cancer subtypes appear more frequently in women with the condition.
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Takeaway: The cancer association was not evenly distributed across ovarian cancers, with clear cell cancer showing by far the strongest long-term increase.
Breast cancer risk was also slightly higher
Ovarian cancer produced the most striking results, but researchers also found a smaller increase in breast cancer diagnoses after the year in which endometriosis or adenomyosis was identified.
The subsequent breast cancer rate was about 1.2 times the expected rate. That increase was far smaller than the association observed for ovarian clear cell cancer, but it remained present across the follow-up period.
The study cannot establish that endometriosis itself caused those breast cancers. Observational studies can identify patterns between conditions, but they cannot rule out every shared hormonal, genetic, reproductive or lifestyle factor that might contribute to both.
That distinction is especially important when relatively common conditions are involved. A modest increase in relative risk can sound alarming without context, even if the actual increase in an individual’s absolute risk remains small.
Takeaway: Breast cancer was modestly more common after an endometriosis or adenomyosis diagnosis, but the study does not show that the condition directly caused those cancers.
The absolute ovarian cancer risk is still low
A sevenfold relative increase can sound frightening, but relative risk and absolute risk answer different questions. Most women with endometriosis will never develop ovarian cancer, including the subtypes highlighted in this study.
The researchers noted previous evidence estimating the lifetime risk of ovarian cancer among women with endometriosis at less than 3%.
That is why the findings do not automatically support removing the ovaries in women with endometriosis simply to prevent cancer. Such surgery has major consequences, including immediate loss of ovarian hormone production when performed before natural menopause.
Routine ovarian cancer screening has also not been shown to reduce mortality in the general population, and the best screening or prevention strategy specifically for women with endometriosis remains uncertain.
The more useful message is awareness. A history of endometriosis can become one piece of information a clinician considers alongside age, family history, genetics, symptoms and other individual risk factors.
Takeaway: Certain relative risks were substantially higher, but ovarian cancer remains uncommon among women with endometriosis, so the numbers should not be interpreted as a prediction that cancer will develop.
Endometriosis may deserve attention beyond pain and fertility
Endometriosis is often discussed mainly in terms of painful periods, pelvic symptoms and difficulty becoming pregnant. Studies like this suggest its long-term health implications may deserve greater attention as well.
The results strengthen evidence that endometriosis and some ovarian cancers may share biological connections, particularly clear cell and endometrioid cancers.
They also highlight why future research needs to identify which women with endometriosis carry the greatest risk. Endometriosis is not one uniform condition, and treating every patient as though she faces the same cancer risk would miss that complexity.
For women living with the disease, the findings are a reason for informed conversations rather than panic. Persistent or unusual symptoms should be evaluated, and a woman’s history of endometriosis should remain part of her long-term medical record even after her reproductive years.
Question for you. If you had endometriosis, would knowing about this association make you want your doctor to discuss your long-term cancer risk more specifically?
