New research suggests a common womb condition may be directly linked to an increased risk of various cancers.
Endometriosis is a disorder in which cells in the lining of the womb grow abnormally in the abdomen and cause scar tissue.
Studies have already shown it is associated with a higher risk of cancer, but doctors have always assumed this is because women with the condition are often infertile.
But now new evidence presented by fertility experts in Lyon, France, indicates that endometriosis itself is a cancer risk.
Researchers claim it makes no difference whether or not women with the complaint have given birth as they are equally likely to develop cancers at an increased rate.
The team from Sweden analysed 63,630 women who had been discharged from hospital after being diagnosed with endometriosis between 1969 and 2002.
Of these, a total of 3,822 went on to develop cancer.
For ovarian cancer the risk was 37% above that of the normal population, for endocrine tumours 38%, for kidney cancer 36% and for thyroid cancer 33%.
Dr Anna-Sofia Melin, from the Karolinska University Hospital in Stockholm, said: "We found that, contrary to what one might expect, endometriosis and nulliparity (childlessness) did not combine to give a higher risk of cancer.
"Maybe the treatment of endometriosis, hormonal or surgical, can influence cancer development."