Breast Cancer Risk Factors

Women in the United States today have about a one in eight (12.6 percent) chance of developing breast cancer by the time they reach their 80s. The risk increases with age: Women in their 30s have a one in 233 (less than half a percent) chance of developing breast cancer, while the risk for women in their 60s is one in 27 (3.7 percent).

Eighty percent of all breast cancers are found in women over the age of 50.

In addition to age and heredity, several other factors appear to slightly increase your risk for developing breast cancer.

  • Having had breast cancer before: If you’ve had cancer in one breast, you’re much more likely to develop it in the other breast.
  • Menstruating at an early age or experiencing later menopause: If you began having menstrual periods before age 12 or went through menopause after age 50, your risk for breast cancer is slightly higher than average. This may be because of the collective amount of the female hormone estrogen your breasts have been exposed to over your lifetime.
  • Your age at your first pregnancy: If you had your first child after the age of 30 or have never had children, you’re at a slightly higher risk for breast cancer. This may be due to the protective changes in breast tissue that occur with full-term pregnancies.
  • Having benign breast disease: Some noncancerous breast conditions may be indicators of an increased risk for breast cancer. These include atypical hyperplasia, in which abnormal cells are overproduced, and lobular carcinoma in situ, in which atypical cells are overproduced in breast lobules. However, having had breast cysts or fibrocystic changes (which cause the breasts to feel lumpy) does not increase the risk of breast cancer.
  • Taking estrogen replacement therapy: Using certain hormonal replacements after the beginning of menopause slightly raises the risk of breast cancer, but the added risk disappears about three to five years after stopping the hormones. Moreover, the risk has been shown to be specific to combination hormone replacement therapy that uses both estrogen and progestin, not therapy involving estrogen alone.
  • Using oral forms of birth control: This raises the risk of breast cancer very slightly, but the increased risk disappears about a decade after you stop taking them.
  • Being overweight or obese: Excess weight increases your risk of breast cancer and the possibility that breast cancer returns after treatment, particularly after menopause. It’s likely that being overweight increases the level of estrogen in the body. Being overweight is defined as having a body mass index (BMI) of 25 or greater; obesity is defined as having a BMI of 30 or greater.
  • Exposure of breast tissue to radiation: This risk factor is particularly associated with exposure during the first two or three decades of your life. The amount of radiation from a mammogram, however, is very small and does not increase your risk.
Understanding Breast Density & Risk
Radiologist Carol Lee answers questions about the concept of breast density and what women should know.
Learn more

Hereditary Breast Cancer

Although a significant number of women with breast cancer have a family history of the disease, only 5 to 10 percent of all cases of breast cancer are due to heredity. You’re two to three times more likely than the general population to develop breast cancer if a first-degree relative (your mother, sister, or daughter) has had the disease.

If more than first-degree relative has had breast cancer, your risk is also higher. This increased risk is even greater if the relatives developed cancer either before the onset of menopause or in both breasts. The presence of certain other types of cancer, such as ovarian cancer and male breast cancer, further increase the possibility that the cancer is hereditary.