Fertility treatment can double the long-term risk of invasive and low-grade ovarian cancers, a study has found.
Stimulating the ovaries of women undergoing in-vitro fertilisation (IVF) increased the chances of patients being diagnosed with ovarian cancer 15 years later.
Overall, ovarian cancer rates were doubled by the treatment which forces the ovaries to produce extra eggs.
The main impact was on non-fatal slow-growing tumours. The risk of developing this condition, known as "borderline ovarian cancer", was raised four-fold.
Although not considered dangerous, borderline ovarian cancer still requires extensive surgery.
Scientists analysed data on more than 19,146 subfertile women who had received at least one ovarian stimulation treatment, and 6,006 subfertile women who did not undergo IVF.
Of 61 women who had ovarian malignancies in the IVF treatment group, 31 had borderline ovarian cancer and 30 had invasive ovarian cancer.
Study leader Professor Flora van Leeuwen, from the Netherlands Cancer Institute in Amsterdam, said: "Our data clearly show that ovarian stimulation for IVF is associated with an increased risk of borderline ovarian tumours and this risk remains elevated up to more than 15 years after the first cycle of treatment."
The risk of potentially deadly invasive ovarian cancer was also raised, but not by a level that was statistically significant.
Prof van Leeuwen said this result may be influenced by how many children, if any, a woman receiving IVF treatment had already given birth to.
"More research is needed to examine the risk of invasive ovarian cancer, especially after a longer follow-up in IVF treated women," she added.
She stressed that the individual risk of developing either invasive or borderline ovarian cancer was "very low".
The research is published online today in the journal Human Reproduction.
Ovarian cancer is the fifth most common cancer among UK women, with more than 6,500 cases diagnosed each year.
It has been called the "silent killer" because often the disease is not detected until an advanced and lethal stage.
Each year around 4,400 British women die from ovarian cancer, a large proportion of those who are diagnosed.
Prof van Leeuwen said the study was now being expanded to include a further 8,800 women who have had their ovaries stimulated as part of IVF.
They would include women who have had three or more treatment cycles.
"If we find out that women who receive several IVF cycles or large doses of ovarian stimulating drugs are at a greater risk of ovarian cancer, then these women would need to be informed about these risks when continuing IVF treatment and possibly advised to discontinue treatment after three to six cycles (depending on which number of cycles would be associated with the high risk of ovarian malignancies)," said Prof van Leeuwen.
British expert Professor Peter Braude, from King's College London, said: "This is an important and worthwhile long term study which goes some way to answering the questions that so many IVF patients ask.
"However the results should be kept in proportion as the increase shown was from around five in 1,000 to seven per 1,000 women. This needs to be balanced against the intention of the treatment; for those infertile to conceive a child.
"The study should be continued and extended to those women who do conceive whose risk may be different from those who do not as has been shown in previous studies."