Breast Cancer Causes and Risk Factors

Medically Reviewed by Sabrina Felson, MD on March 24, 2023
4 min read

The precise causes of breast cancer are unclear, but we know the main risk factors. Still, most women considered at high risk for breast cancer do not get it, while many with no known risk factors do.

The main risks are being older and having breast cancer in your family. The risk goes up for women with certain types of benign breast lumps and for women who have had ovarian cancer. And if you've had breast cancer, you can get it again.

In 1940, the lifetime risk of a woman developing breast cancer was 5%. Now the risk is about 13%. In about half of the cases, the woman has no known risk factors.

Being a woman. Men can get breast cancer too, but it’s 100 times more likely to affect women.

History of breast cancer. A woman who has had cancer in one breast, such as ductal carcinoma in situ (DCIS) or invasive breast cancer, 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.

Age. Your risk goes up as you age. About 77% of women diagnosed with breast cancer each year are over 50, and more than 40% are 65 and older.

In women ages 40 to 50, there is a 1 in 68 chance of developing breast cancer. The highest-risk age group is those 50 to 60. t

Direct family history. Having a mother, sister, or daughter ("first-degree" relative) with breast cancer puts a woman at higher risk. It's even greater if this relative developed breast cancer before 50 and had cancer in both breasts.

Having one first-degree relative with breast cancer roughly doubles your risk, and having two first-degree relatives triples your risk. Having a male blood relative with breast cancer will also increase the risk.

Genetics. About 5% to 10% of breast cancer cases are inherited. Carriers of alterations in either of two genes, called BRCA1 or BRCA2, are at higher risk. Women with an inherited alteration in the BRCA1 gene have a 72% chance of developing breast cancer by the time they’re 80. There’s a 69% chance that a woman with an inherited alteration in the BRCA2 gene will get breast cancer by that age.

Dense breasts. Your breasts are a mix of fatty, fibrous, and glandular tissue. Dense breasts have more glandular and fibrous tissue and less fat. A woman with dense breasts is 1.5 to 2 times more likely to get breast cancer.

Breast lesions. Having atypical hyperplasia (lobular or ductal) or lobular carcinoma in situ increases a woman's breast cancer risk by four to five times.

Distant family history. This refers to breast cancer in second- or third-degree relatives such as aunts, grandmothers, and cousins.

Previous abnormal breast biopsy. Women with earlier biopsies showing any of the following have a slight increased risk: fibroadenomas with complex features, hyperplasia without atypia, sclerosing adenosis, and solitary papilloma.

Reproductive history. The more estrogen your body has made over time, the higher your risk. Getting your period before age 12, starting menopause after age 55, and never being pregnant raise your lifetime exposure to estrogen and breast cancer risks.

Radiation treatment. If you had radiation treatment to your chest before age 30, usually as treatment for cancers such as lymphoma.

Other cancer in the family. If a family member had ovarian cancer before age 50, your risk is increased.

Heritage. In the U.S., white and African-American women are more likely to get it than Hispanic, Asian/Pacific Islander, and Native American women.

Exposure to diethylstilbestrol (DES). Many women received this drug between 1940 and 1971 to prevent miscarriage. If you or your mother got it, your odds of breast cancer go up.

Weight. Being overweight after menopause increases your odds.

Drinking alcohol.Alcohol is linked to breast cancer. Compared with nondrinkers, women who drink one alcoholic drink a day have a very small increase in risk, and those who are moderate drinkers (2 to 3 drinks a day) have about a 20% higher risk.

Hormone replacement therapy (HRT). Long-term use of estrogen and progesterone increases the risk of breast cancer. This risk seems to go away if you've stopped using them for 5 years or longer.

Being inactive. Your odds go up if you don’t exercise.

Reproductive history. Having your first child after age 30 or never having a full-term pregnancy puts you at higher risk. So does not breastfeeding.

These things do not affect your risk of breast cancer:

  • Using antiperspirants
  • Wearing underwire bras
  • Having an abortion or miscarriage
  • Having fibrocystic breast changes (dense breast tissue with benign cysts)
  • Multiple pregnancies
  • Coffee and caffeine
  • Using hair dye