But the findings, based on a study of more than 21,000 women and published in the journal Fertility and Sterility, cannot determine whether IVF contributed to the cancers or whether something else could explain the risk.

"I don't think it's a huge increased risk that you should worry or panic (about)," said Louise Stewart, the study's lead author and a researcher at the University of Western Australia in Crawley.

She added, however, that the findings did show a link between the two and doctors should keep that in mind.

For the study, Stewart and her colleagues collected information on 21,025 women between the ages of 20 and 40 who went through fertility treatment at the hospitals of Western Australia between 1983 and 2002.

They were able to piece together enough data to follow the women for some 16 years to see if they developed breast cancer.

Roughly 1.7 percent of the 13,644 women who only used fertility drugs without IVF ended up developing breast cancer by the end of the study. That figure was about two percent for women who used fertility drugs and underwent IVF - a difference that researchers said wasn't statistically significant.

When they divided the women into different age groups, though, that changed.

Women who started taking fertility drugs around their 24th birthday and went through IVF had a 56 percent greater chance of eventually developing breast cancer compared to those in the same age group who only went through fertility treatments without IVF.

But there was no increased risk for women who started fertility treatments when they were about 40 years old, regardless of whether they had IVF or not.

Stewart told Reuters Health that a possible reason that younger women see an increased risk of breast cancer is that they are exposed to higher levels of circulating estrogen during their cycles of IVF treatment.

"The development of breast cancer is linked to estrogen exposure and the longer one is exposed, the greater the risk," said Linda Giudice, president-elect of the American Society of Reproductive Medicine, in a statement.

"In an IVF cycle there is a short, but significant elevation in circulating estrogen, and whether this is linked to the observations found in the study is not clear at this time."

Stewart added that another explanation could be that younger women who undergo IVF may be different in some significant way from those who only have other types of fertility treatment.

"If, for example, younger women who had IVF were more likely to have a specific cause of infertility, and this was related to an increased risk of breast cancer, then it would appear that IVF was related to breast cancer when in fact it was the type of infertility that was more common in women who had IVF," she said.

She emphasized that this is just speculation, adding that the data used in the study didn't include information about the women's causes of infertility - and that the current study's results need to be confirmed by future research.

SOURCE: bit.ly/MDKsYi

(Reporting from New York by Andrew Seaman at Reuters Health; Editing by Elaine Lies and Ed Lane)

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