Earlier this month, I wrote about what not to say to someone diagnosed with cancer. One of the things on that list was asking if “they” know what caused it. Early in my cancer journey, my doctor told me that, unlike some other cancers, most women diagnosed with breast cancer have no known risk factors beyond being a woman. Today, I’m taking a look at those known breast cancer risk factors and some new emerging risks that you may not have heard of.

This post was originally published in October 2021 and updated in December 2024.

Breast cancer is one of the most common cancers in Canadian women. The Public Health Agency of Canada reports 1 in 9 women in Canada will be diagnosed with breast cancer in her lifetime.

Male breast cancer represents less than 1% of breast cancer cases in Canada. Although I acknowledge that male breast cancer is a concern, this post will focus on risk factors for female breast cancer.

While the exact cause of breast cancer is still unknown, let’s look at the risks.

It’s important to make sure medical information comes from a reputable source. The information in this post comes from the Public Health Agency of Canada and Breastcancer.org—two excellent and credible sources of information on breast cancer.

Breast cancer risk factors

The risk factors fall into two main areas—biological risks and lifestyle risks.

Biological risk factors

Biological risks are mostly out of our control.

The two biggest risk factors—age and sex—are things we can’t change. The single biggest risk is being a woman, and the risk increases with age with most cases diagnosed after age 50.

Although other biological factors increase the risk, the increase is small in comparison to the big two.

Family history is also a factor. Having a close relative—a sister, mother, or daughter—diagnosed with breast cancer doubles your risk. Two close relatives with breast cancer makes your risk 5 times higher than average.

Just 5% to 10% of breast cancers are hereditary meaning they are caused by abnormal genes passed on from parent to child. Most inherited breast cancer cases relate to mutations in the BRCA1 and BRCA2 genes.

Race and ethnicity also play a role. White women have a slightly higher risk of breast cancer, but black women are more likely to be diagnosed at a younger age with more aggressive, more advanced stage breast cancer.

And we should also consider reproductive factors. Never having a full-term pregnancy or having a first child after age 30 poses a small increased risk. Similarly, not breastfeeding or breastfeeding for less than one year poses a slightly higher risk than for women who breastfeed for a year or more.  

Women who started menstruating before age 12 or reached menopause after age 55 also see a slightly elevated risk.

Lifestyle risk factors

While there’s not much we can do about the biological factors, we can take steps to reduce our risk by living a healthy lifestyle.

Lifestyle risks fall into five main areas. There are no surprises on this list—weight, exercise, diet, smoking, and alcohol consumption. Not only are these factors linked to increased breast cancer risk, but they are also risk factors for many other diseases.

Let’s take a closer look.

• Weight gain in adult life can significantly increase the risk of breast cancer. In fact, gaining just 5 kilograms (about 12 pounds) after menopause has been associated with up to 12% increased risk of breast cancer.
• Increased physical activity can reduce breast cancer risk by 25-30%. That’s huge! Check out What you need to know about exercise and cancer for more information on how much exercise you need.
• There is research to suggest a poor diet may increase your risk of breast cancer. Eating a balanced diet, and reducing fat consumption, can help in this area.
• Nearly 17% of all deaths in Canada can be attributed to smoking. Although not usually considered a risk factor for breast cancer, some recent studies have found a connection between tobacco use and breast cancer.
• Finally, drinking alcohol may increase your chance of getting breast cancer by over 20%. If you drink, limit your alcohol intake to one drink per day.

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Emerging risks

Research into breast cancer risk factors reveals new information all the time. Breastcancer.org reports some emerging risks that are worth taking note of.

• Low vitamin D levels. Living in a northern climate, this is a concern in the wintertime because much of our vitamin D comes from sunlight. Just 15 minutes of sun exposure, three times a week, can be enough to give you the recommended level of vitamin D.  
• Light exposure at night. Women who work at night have higher levels of breast cancer than those who work during the day. Researchers believe this is linked to melatonin levels, but more study is needed.
• Exposure to chemicals in food, cosmetics, lawn/garden products, plastic, sunscreen, and water. This is a big area of concern for me, and I work hard to limit my exposure to chemicals.

The exact cause of breast cancer is still unknown, but it’s important to know the risk factors and what you can do to reduce your chances of getting this disease.

Lastly, while screening won’t prevent cancer, early detection can improve the outcome significantly.

Please check the screening guidelines in your area and stay on top of your regular checkups and mammograms. It could save your life.

Did any of the breast cancer risk factors on this list surprise you? Tell me your thoughts below.