In recent years, some of the same Dutch scientists involved in the breast cancer study have released research suggesting that women who undergo IVF aren't more likely than the general population to develop colon or endometrial cancer.
As in many complicated medical issues, experts say, more research is needed, especially on the topic of IVF and ovarian cancer. Last fall, British researchers published a large study that found that women who undergo IVF are over a third more likely to develop ovarian cancer than those who don't get the treatment.
At the time, those researchers cautioned that the connection might be partly caused by the patients' underlying infertility — which is itself is a risk factor for ovarian cancer — rather than the treatment itself. And many scientists agree with that.
"I don't think there is a direct link between IVF and ovarian cancer," said Terri Woodard, assistant professor in the Department of Gynecologic Oncology and Reproductive Medicine at MD Anderson Cancer Center. "There are many things that confuse the issue."
Mia Gaudet, the strategic director of breast and gynecologic cancer research at the American Cancer Society, said the ovarian-cancer issue remains "an open question."
Indeed, questions have swirled around IVF for decades because the treatment regimens require elevating certain hormones to levels far above normal, and some of those hormones are known to affect cancers. For years, studies have come down on one side, then the other, on whether IVF is linked to breast cancer. The result has been a worrisome muddle for patients.
The new study was published Tuesday in the Journal of the American Medical Association and was conducted by researchers at the Netherlands Cancer Institute. It involved more than 25,000 women with a median follow-up of more than 20 years.
"I was very excited about it," Woodard said. She said her patients routinely ask her whether their risk of breast cancer will rise, and now "we have a general, population-wide answer that says, no, we don't think so."
She expressed skepticism about one of the findings: That women who underwent seven or more IVF cycles had a lower risk of breast cancer than those who underwent one or two cycles.
"That doesn’t make me want to run and do seven cycles," she said. "I think you should do a couple of cycles — two to three, and then I'm out."
Gaudet, in assessing the breast cancer study, was encouraged but cautious. "This is a very reassuring first step," she said. "But it's not conclusive."
For one thing, she said, the medications and treatment protocols have changed over the years, and so the risks might have changed, as well.
In addition, while the study extended over a long period of time, many of the oldest women still haven't reached the age of highest incidence for breast cancer. "They'll have to continue to be monitored to see if they develop breast cancer later in life," she said.