What are the risk factors for breast cancer?

The strongest risk factors for breast cancer are female sex and age. As mentioned above, breast cancer is far more common in women. However, men can also get breast cancer, but it is 100 times more common in women than in men. The risk of breast cancer increases with age, and breast cancer incidence rates are highest in women over 70 years of age. Only about 5% of women in the U.S. who are diagnosed with breast cancer are under 40 years old.

Family history and genetics are also risk factors. A woman is more likely to develop breast cancer if she has immediate family members who are affected. It is also possible to inherit gene mutations that increase the risk of breast and other cancers. Inherited mutations, however, account for only 5% to 10% of all breast cancers. BRCA1 and BRCA2 genetic mutations are associated with a markedly increased risk of breast cancer. Other genetic mutations have also been linked to an increased incidence of breast cancer.

A personal history of breast cancer increases a woman's risk of developing another breast cancer. A woman is also at increased risk if she has had Hodgkin's disease or other cancers. Having dense breasts (a greater proportion of glandular and connective tissue in the breasts relative to fat) means that a woman has a four- to five-fold increase in risk for breast cancer. Certain benign breast conditions may also increase a woman's risk (for example, atypical hyperplasia, a condition in which abnormal cells proliferate within the breast ducts).

Radiation treatment during youth, for example, chest radiation for treatment of another cancer, increases a woman's risk of breast cancer later in life.

Not having given birth or having given birth to a first child after the age of 35 are associated with moderate increases in breast cancer risk. Overweight and obesityafter menopause also increase risk. Having begun menstruating before age 12 or having reached menopause after age 55 are associated with slight increases in breast cancer risk, likely because of the longer duration of estrogen exposure. Current or recent use of postmenopausal hormone therapy or birth control pills seems to slightly raise a woman's risk of breast cancer.