Women given fertility drugs to produce eggs had more than triple the risk of an ovarian tumour that may turn cancerous, say Dutch researchers.
But the absolute risks are very low, they add.
A cancer charity said numbers involved in the study, published in the journal Human Reproduction, were too small to draw firm conclusions.
The study tracked more than 25,000 women attending IVF clinics in The Netherlands in the 80s and 90s.
Follow-up investigations revealed more cases than expected of ovarian tumours in women who had gone through IVF, which involves stimulating the ovaries to make eggs.
The biggest increase was in a type of growth, known as a borderline ovarian tumour, which can sometimes turn into cancer. It is less aggressive than other types of ovarian tumour, but requires surgery.Continue reading the main story
Women should be informed about this but the risk should not be overstated”End Quote Prof Flora van Leeuwen Netherlands Cancer Institute, Amsterdam It normally affects around one in 1,000 women in the general population, but was found in about 3.5 in 1,000 women who had gone through IVF, say the researchers.
A smaller increase in other types of ovarian tumour was also found. Overall, ovarian cancer rates were twice as high among women who had gone through fertility treatment, the experts said.
Prof Flora van Leeuwen, a co-author of the study, told the BBC: "The absolute risk of these tumours is very low. But there is an increased risk of a borderline malignant tumour that needs surgery.
"Women should be informed about this but the risk should not be overstated."
Another co-author, Prof Curt Burger added: "The main message is that women who have had IVF shouldn't be alarmed. The incidence of ovarian cancer was extremely low."'Reassuring'
Further research is planned to confirm the finding in a larger number of patients, and to look at whether some women are more at risk.
At present, the numbers involved are small. There were 61 women with ovarian tumours in the IVF treatment group; 31 had borderline ovarian tumours and 30 had ovarian cancer.Continue reading the main story Ovarian cancer is the 5th most common cancer in women in the UKMost cases are in women who are past the menopauseRisk factors include a family history of cancer, being infertile or having fertility treatment, and smokingThe symptoms of ovarian cancer can be very vague, particularly when the disease is in its early stages.Early symptoms can include pain in the lower abdomen or side, and/or a bloated, full feeling in the abdomenSource: Cancer Research UKCommenting on the study, Prof Hani Gabra, of the Ovarian Cancer Action Research Centre at Imperial College London, said:
"Reassuringly, and in keeping with lots of previous research in this area, this study shows that the risks of invasive ovarian cancer are small in populations of patients receiving ovarian stimulation for IVF.
"Although this study shows that ovarian stimulation may increase the risk of much less aggressive borderline ovarian tumours, it underlines the fact that ovarian stimulation for IVF is not a major risk factor for invasive ovarian cancer."
Dr Claire Knight, senior health information officer at Cancer Research UK, said: "This interesting study suggests a possible link between ovarian stimulation for IVF and borderline ovarian tumours, but it certainly doesn't show that IVF causes invasive ovarian cancer.
"There were only a relatively small number of cases in this study, and the researchers didn't find that risk increased with the number of cycles a woman had, making conclusions hard to reach.
"Women can reduce their risk of ovarian cancer by being a non-smoker and keeping a healthy weight, and women who have taken the Pill or been pregnant are also at lower risk."