Women getting fertility treatments can be reassured that in vitro fertilisation (IVF) does not increase their risk of breast and gynaecological cancers, according to a new study of Israeli women.
"The findings were fairly reassuring. Nothing was significantly elevated," said lead author Louise Brinton, chief of the Hormonal and Reproductive Epidemiology Branch at the National Cancer Institute in Rockville, Maryland.
Still, she added, "We should continue to monitor these women."
IVF treatments can involve ovulation-stimulating drugs or ovary puncturing to collect eggs - procedures that researchers have suspected may increase women's risk of cancer.
Indeed, previous studies linked IVF early in life to heightened risks of breast cancer and borderline ovarian tumours.
But other studies found little connection between fertility treatments and cancer.
The association has been difficult to untangle, experts said, in part because it's hard to know whether unmeasured factors not related to IVF itself may affect the risk of cancer in women who have trouble conceiving.
And so far, there haven't been a lot of women who developed cancer after fertility treatment included in studies.
"We all want answers, but it's a very difficult exposure to study, particularly when we don't have the numbers we would really like," Brinton told Reuters Health.
To boost those study figures, she and her colleagues examined the medical records of 67,608 women who underwent IVF treatments between 1994 and 2011 and 19,795 women who sought treatment but never received IVF.
The researchers linked those women's files to a national cancer registry and found 1509 of them had been diagnosed with cancer through mid-2011.
There was no difference in women's chances of being diagnosed with breast or endometrial cancer based on whether they were treated with IVF.
The researchers did find a woman's risk of ovarian cancer slightly increased the more rounds of treatment she received, they wrote in the journal Fertility and Sterility. But that finding could have been due to chance.
Brinton said her study was too small to conclusively link IVF and ovarian cancer - and that it remained very rare, with 45 cases in the entire study.
A similar association was found in a study headed by Dr Bengt Källén, director of the Tornblad Institute at Lund University, Sweden . Källén, who was not involved in the current research, said any increased ovarian cancer risk might be due to the dysfunctional ovaries themselves.
"Infertile women have a primary problem with their ovaries and IVF has nothing to do with it," Källén told Reuters Health. "It's a rather difficult thing to disentangle if there is an effect from the hormones or from the IVF procedure."
Dr Sherman Silber of the Infertility Center of St Louis warned these types of studies have several biases that could make the results difficult to interpret.
For example, women undergoing IVF are watched extra closely, which would likely increase the chance that ovarian cancers are detected, said Silber, who also was not involved in the new study.
"You have to be extraordinarily cautious about this kind of a study," Silber told Reuters Health. "If anything, it's reassuring. One doesn't see any real increase in cancer."
Researchers said future studies would require larger groups of IVF-treated women to tease out potential long-term risks.
"What is surprising all of us who are working in this area is how almost every study gets a different answer," Brinton said. "There's a need for monitoring, but we also shouldn't get too alarmed at this early stage."