What Causes Breast Cancer? The 18 Factors That Put You at the Biggest Risk

With over 200,000 U.S. women diagnosed with breast cancer a year, breast cancer is the most common type of cancer in women. Breast cancer occurs when cells in the breast begin to grow abnormally, dividing uncontrollably and building up into a mass that usually is detected as a hard lump. But what causes that to happen? Experts still aren’t entirely sure what exactly is the initial spark that triggers those cell mutations that cause breast cancer—but they do know that several risk factors up your chances.

Those risk factors can be broken down into two categories: lifestyle and genetic. “When we think about breast cancer, we try to break it down into things you can and can’t change,” says Megan Kruse, M.D., an oncologist at the Cleveland Clinic in Ohio and assistant professor of medicine at the Cleveland Clinic Lerner College of Medicine. “There are risk factors you’re born with and then there are others that you can actually do something about.”

Know this: just because you have one or more breast cancer risk factors, that doesn’t necessarily mean you’ll develop it. And on the flip side, even if you have none of the following risk factors, that doesn’t mean you’re 100% in the clear (though knowing them can help you in your efforts to prevent breast cancer). All that said, here are the biggest lifestyle and genetic factors that can put you at risk of developing breast cancer.