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

Even a few drinks a week is linked with an increased risk of breast cancer in women. According to the American Cancer Society, women who drink two or three drinks a day have a 20% higher risk of developing breast cancer compared to women who don’t drink alcohol. Alcohol can raise estrogen levels in the body, which may be why it increases your risk.

“Obesity is a risk factor particularly among postmenopausal women,” Dr. Kruse says. Before menopause, your ovaries make most of your estrogen. After menopause, the ovaries stop making estrogen, so most of the hormone comes from fat tissue—having too much fat can raise estrogen levels and increase your risk of getting breast cancer. Additionally, women who are overweight tend to have higher blood insulin levels, which have been linked to breast cancer. “The closer a woman is to her ideal body weight, the less risk she has of getting breast cancer,” Dr. Kruse notes.

Regular physical activity reduces breast cancer risk, especially in women past menopause. Though how much activity you need is unclear, some studies have found that even just a few hours a week of exercise may be helpful (though more is likely better). A study published in JAMA examined the presence of breast cancer in active women and less active women and found that women who engaged in regular strenuous physical activity at age 35 had a 14% decreased risk of breast cancer.

“We see a slightly increased risk of breast cancer among women who have either never had a child or had their children after the age of 30,” Dr. Kruse says. That said, the effect of pregnancy seems to depend on the type of breast cancer you have. For example, having a type of breast cancer called triple-negative seems to increase risk.

Some studies suggest that breastfeeding may slightly lower breast cancer risk, especially if it’s done for one and a half to two years. This may be because breastfeeding reduces a woman’s total number of lifetime menstrual cycles and making milk limits a breast cells' ability to act abnormally. A study published in the Journal of the National Cancer Institute examined breast tissue from women with breast cancer and found that women who had breastfed had a 30% lower risk of recurrence and a 28% lower risk from dying of breast cancer.

The hormones in some birth control methods including oral contraceptives, birth control shots, and IUDs may increase breast cancer risk. A study published in the New England Journal of Medicine studied 1.8 million women between the ages of 15 and 49 and found a 20% increased risk of developing breast cancer among women who use hormonal birth control.

Hormone therapy with estrogen and progesterone may help relieve symptoms of menopause and help prevent osteoporosis, but it can also increase the risk of developing and dying from breast cancer by about 75%. It can also increase the likelihood that the cancer is found at a more advanced stage and up the risk of heart disease, blood clots, and strokes. According to the American Cancer Society, the risks may outweigh the benefits of hormone therapy so be sure to talk to your doctor before using it.

Since breast cancer is a hormonally-driven cancer—it typically needs estrogen in order to grow—simply being a woman is probably the biggest risk factor for developing breast cancer, says Jennifer Specht, M.D., an oncologist at Seattle Cancer Care Alliance and an associate member of the clinical research division at Fred Hutchinson Cancer Research Center.

The risk of breast cancer increases with age. Most breast cancers are found in women age 55 and older.

About 5 to 10% of breast cancer cases are thought to be hereditary, meaning they are the result of gene defects (called mutations) passed on from a parent. Specifically, having an inherited mutation of the BRCA1 or BRCA2 gene is the most common cause of hereditary breast cancer. (Here's what you should know about BRCA testing.)

The American Cancer Society stresses that most women (about eight out of 10) who get breast cancer do not have a family history of the disease. However, having a first-degree relative (mother, sister, or daughter) with breast cancer almost doubles your risk, and having two first-degree relatives triples your risk. Overall, less than 15% of women with breast cancer have a family member with the disease.

If you’ve had cancer in one breast, you have a higher risk of developing new cancer in the other breast or in another part of the same breast. Although the risk is low, it tends to be higher for younger women with breast cancer.

White women are slightly more likely to develop breast cancer than Black women; however, breast cancer is more common in Black women under the age of 45, and Black women are more likely to die from breast cancer at any age. Asian, Hispanic, and Native American women have both a lower risk of developing and dying from breast cancer.

Breasts are made up of fatty tissue, fibrous tissue, and glandular tissue. You might be told you have “dense breasts” if a mammogram shows you have more glandular and fibrous tissue and less fatty tissue. Women with dense breasts are 1.5 to 2 times more likely to develop breast cancer than women with average breast density, according to the American Cancer Society.

If you got your period before the age of 12, that means you’ve experienced more menstrual cycles in your lifetime and have been more exposed to estrogen and progesterone, which increase the risk of breast cancer.

Conversely, going through menopause after age 55 means you’ve had more menstrual cycles, which again prolongs your exposure to estrogen and progesterone, increasing your risk of breast cancer.

“Being treated for a childhood cancer with radiation to the chest significantly increases the risk of developing breast cancer later in life,” Dr. Specht says. The risk is highest if you had radiation as a teen or young adult when your breasts were still developing.

Women who took diethylstilbestrol (DES), an estrogen-like drug administered from the 1940s through the early 1970s to lower the chances of miscarriage, have a slightly increased risk of developing breast cancer. Women whose mothers took DES during pregnancy may also have a slightly higher risk of breast cancer, according to the American Cancer Society.