Infertile women are more likely to have "dense breasts" - a significant risk factor for breast cancer, a new study suggests.
Infertility and some infertility treatments are associated with small but significant increases in breast density, researchers said.
Women who are deemed to have ''high breast density'' are at a significantly higher risk of developing breast cancer than others.
The authors from the Karolinska Institutet, Sweden, studied 43,313 women aged between 40 and 69 years who had mammograms between 2010 and 2013.
There were 8,963 women who reported fertility problems and, of these, 1,576 had undergone controlled ovarian stimulation (COS), 1,429 had had hormonal stimulation without COS and 5,948 had received no fertility treatment.
The researchers then first compared mammographic density levels between fertile and infertile women. They then compared infertile women who had never received hormonal fertility treatment to those who had.
The research, published in the journal Breast Cancer Research, found that women with a history of infertility had denser breasts than other women.
The association was more pronounced in women who had undergone COS , the hormone treatment required for IVF and some other fertility treatments.
"The results from our study indicate that infertile women, especially those who undergo COS, might represent a group with an increased breast cancer risk," said lead author of the study Frida Lundberg.
"While we believe it is important to continue monitoring these women, the observed difference in breast tissue volume is relatively small and has only been linked to a modest increase in breast cancer risk in previous studies."
Commenting on the study, Paul Pharoah, professor of cancer epidemiology at the University of Cambridge, said: "The investigators found that infertility and some infertility treatments were associated with small but significant increases in breast density, even after controlling for other factors associated with breast density such as body mass index and pregnancies. This finding suggests that infertility treatment might be associated with an increased risk of breast cancer.
"However, this study found no direct link between infertility/infertility treatment and breast cancer risk. This is a cross-sectional study - meaning the breast density was measured at the same time as the history of infertility was ascertained. It is not possible to know whether the infertility/treatment resulted in an increase in breast density or whether women with dense breasts are more likely to have dense breasts."
Dr Emma Blamont, senior research officer at the charity Breast Cancer Now, added: "If confirmed, this is a link that could help women who have undergone fertility treatment better understand their risk of breast cancer. The study suggests that women with a history of infertility might have denser breasts, a factor known to increase one's risk of developing breast cancer.
"The next step is to identify whether it is the original infertility or the hormone treatment for it that is causing this effect. Further studies will be required to fully understand the link between fertility and breast density before any recommendations can be made for women.
"Importantly, the best way for women to reduce their risk of breast cancer is to maintain a healthy weight, be more physically active and reduce their alcohol intake."
Samia al Qadhi, chief executive of the charity Breast Cancer Care, added: "This thought-provoking early insight suggests infertility and its treatment could mean a slight increase in breast density, which is a known risk factor for developing breast cancer.
"However, there is no evidence within this study to show these women will definitely go on to develop breast cancer. This area of research has a long way to go before we can find all the answers.
"The take-home message for women today is if you've had hormonal fertility treatment this does not mean you will definitely have a higher risk of breast cancer.
"The biggest risk factors for the disease remain out of our control - being female and getting older."
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