The findings, however, cannot determine whether IVF contributed to the cancers or whether something else could explain the link.

"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 her findings did show a link between the two and doctors should keep that in the back of their minds.

For the new study, Stewart and her colleagues collected information on 21,025 women between the ages of 20 and 40 years old 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.

When all the women were analyzed together, the researchers did not see an overall link between IVF and breast cancer.

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

When they divided the women into two different age groups, however, 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.

Meanwhile, 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, according to findings published in Fertility and Sterility.

Stewart told Reuters Health that the most likely reason the 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 Dr. 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 this study is not clear at this time," Giudice said.

Stewart added, 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 treatments.

"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," Stewart explained.

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

Giudice, who was not involved with the new study, added, "Women should be reassured that, overall, IVF was not associated with an increased risk for development of breast cancer."

SOURCE: bit.ly/MDKsYi Fertility and Sterility, online May 28, 2012.

• A Permanent Economic Emergency