Risk Factors for Breast Cancer In 1940, the lifetime risk of a woman developing breast cancer was 5%, or one in 20. The American Cancer Society estimates that risk to be 13% in 2012, or more than one in 8. In many cases, it's not known why a woman gets breast cancer. In fact, 75% of all women with breast cancer have no known risk factors.

What Are the Risk Factors for Breast Cancer?

A risk factor is anything that increases a person's chance of getting a disease. Different cancers have different risk factors.

However, having a cancer risk factor, or even several of them, does not necessarily mean that a person will get cancer. Some women with one or more breast cancer risk factors never develop breast cancer, while most women with breast cancer have no apparent risk factors.

Significantly higher risk

A woman with a history of cancer in one breast is three to four times likelier to develop a new breast cancer, unrelated to the first one, in either the other breast or in another part of the same breast. This is different than a recurrence of the previous breast cancer.

Moderately higher risk

• Getting older. Your risk for breast cancer increases as you age. About 77% of women diagnosed with breast cancer each year are over age 50, and almost 50% are age 65 and older. Consider this: In women 40 to 49 years of age, there is a one in 68 risk of developing breast cancer. In the 50 to 59 age group, that risk increases to one in 37.
• Direct family history. Having a mother, sister, or daughter ("first-degree" relative) who has breast cancer puts a woman at higher risk for the disease. The risk is even greater if this relative developed breast cancer before menopause and had cancer in both breasts. Having one first-degree relative with breast cancer approximately doubles a woman's risk, and having two first-degree relatives increases her risk fivefold. Having a male blood relative with breast cancer will also increase a woman's risk of the disease.
• Genetics. Carriers of alterations in either of two familial breast cancer genes called BRCA1 or BRCA2 are at higher risk. Women with an inherited alteration in either of these genes have up to an 85% chance of developing breast cancer in their lifetime.
• Breast lesions. A previous breast biopsy result of atypical hyperplasia (lobular or ductal) or lobular carcinoma in situ increases a woman's breast cancer risk by four to five times.