“Infertile women, especially those who undergo controlled ovarian stimulation (COS), might represent a group with an increased breast cancer risk,” Dr. Frida Lundberg, a researcher at Karolinska Institutet in Sweden, said in a recent statement.

“Might represent” — two important words here.

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Dr. Lundberg just wrapped up a study of more than 43,000 women that found infertility and in vitro fertilization (IVF) are linked to higher breast tissue density in women — a known risk factor for breast cancer.

Women with a history of infertility had denser breasts compared to other women. That link was more pronounced in women who had undergone an IVF treatment known as controlled ovarian stimulation (COS). Study participants were between 40 and 69 years old; breast cancer is typically diagnosed over the age of 50.

The authors say their findings could show a potentially adverse effect of COS on breast density, but that correlation may be due to an individual’s underlying infertility issue — perhaps not the treatment itself.

“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,” Lundberg stated.

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Can IVF Cause Breast Density?
The researchers acknowledged some limitations of the study: Women who reported infertility may be fertile women with infertile partners. The study did not take into account timing and how many treatments a woman had undergone.

The researchers also do not know if higher breast density was only found in women who had recently received treatment. They did an analysis by age, and found a stronger association between COS and mammographic density in older women, something that could show a long-term effect of COS on breast tissue.

Breast tissue is composed of two types of tissue: dense, fibroglandular and non-dense, fatty tissue. Research has found that women with extremely dense breasts have a four- to six-fold higher risk for breast cancer compared to women with non-dense breasts.

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Hormone therapies such as COS boost estrogen and progesterone levels and have been suspected of raising breast cancer risk.

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IVF is Linked, but Not a Cause of Breast Density, Cancer
Dr. Charles L. Shapiro, who leads breast cancer research at the Mount Sinai Health System in New York, said he has not noticed a correlation between fertility treatments and dense breast tissue.

“The consensus in 2016, based primarily on prospective cohort studies and registry studies, is that infertility treatments or in vitro fertilization does not cause an increase in the risk of developing breast cancer,” he said.

“IVF does not seem to cause any other cancers,” Shapiro said. He said the main side effects of fertility treatments can include skin irritation at injection sites, abdominal bloating, headaches, breast tenderness, and nausea.

IVF often results in multiple births, which have risks to the mother and babies, he added.

Dr. Serena Chen, a fertility specialist with the Institute for Reproductive Medicine and Science at Saint Barnabas in New Jersey, said she screens patients prior to treatment — including all women over 40.

“Dense breasts are extremely common,” she told LifeZette, adding that all patients with dense breasts then have a sonogram or MRI.

“For the vast majority, the benefits of IVF outweigh the risks,” she said, adding that it’s the most effective fertility treatment and can shorten exposure to hormones in some ways because most people can conceive within one or two cycles. She doesn’t believe the research will change people’s choices to use IVF, especially because research has shown that pregnancy and breastfeeding are protective against breast cancer.

A 2013 study found that women who have multiple babies after IVF are at a higher risk for breast cancer compared to those who have one child or none at all. In that study, researchers said multiple pregnancies may not be to blame — other traits could put them at risk.