Women using IVF are a third more likely to develop ovarian cancer, the biggest study of fertility treatment in the world has discovered.
Scientists at University College London said underlying problems in infertile women may be driving the increased risk, but warned the finding leaves open the possibility the procedure itself might be to blame.
Previous studies suggested ovarian stimulation used to harvest eggs could fuel cancer but most specialists dispute the danger. However, British experts said the new findings were serious enough to consider regular screening of IVF patients.
They called for infertile women to be informed the risk of ovarian cancer was higher than in natural conception.
Researchers studied every IVF procedure recorded by the Human Fertilisation and Embryology Authority in Britain from 1991 to 2010 - more than 250,000 women.
Professor Alastair Sutcliffe of the Institute of Child Health at UCL said the research provided mixed news. "Compared to other women in the UK of the same age range and time frame, we found the rates of breast and uterine cancer were no different to UK women as a whole." But there was an increased risk of ovarian cancer, he said.
The authors of the report conclude: "Certain results argue against an association with assisted reproductive technology itself, but others leave open the possibility it might affect risk."
The risk is still small: just 15 in every 10,000 patients developed ovarian cancer in the study period. The figure is around 11 in 10,000 generally.
Professor Geeta Nargund, medical director of Create Fertility which has five clinics in the UK, said the findings were concerning. "We should be moving towards milder stimulation and fewer drugs."
Dr Adam Balen, professor of reproductive medicine at the University of Leeds, said: "This study, from a huge database, suggests that women who have IVF with certain conditions, such as endometriosis, may be at increased risk of developing ovarian cancer."
But charities said IVF patients should not be overly alarmed.
Dr Julie Sharp, head of health information at Cancer Research UK, said: "This is important research, but doesn't prove fertility treatment increases ovarian cancer risk."
The research was presented at the annual conference of the American Society for Reproductive Medicine in Baltimore.