Women in the United States today have about a one in eight (12.6 percent) lifetime chance of developing breast cancer by the time they reach their 90s. 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). Only by adding all these yearly risks do we arrive at the widely cited one in eight statistic. The key point is that the actual risk in any year of life is much lower than one in eight for most people.
The overall aging of the US population means that the rising risk for breast cancer that comes with getting older will have greater impact in the future. About 80 percent of all breast cancers are found in women over the age of 50. As this age group increases in number, we can expect more breast cancer cases to be diagnosed.
In addition to age and heredity, several other factors appear to slightly increase your risk for developing breast cancer.
- Previous Breast Cancer: If you’ve had cancer in one breast, you’re more likely than the average woman to develop a new cancer in the other breast. In recent years the risk of a second breast cancer has decreased, however. This is because many drugs — especially the various anti-estrogen drugs — used for breast cancer treatment also reduce the risk of developing new breast cancers.
- Early Menstruation or Late 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 the average woman’s. This may be because of the amount of the female hormone estrogen that your breasts have been exposed to over your lifetime. But the precise cause is not known.
- Age at 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.
- Benign Breast Disease: Some noncancerous breast conditions may increase your risk for breast cancer. These include atypical hyperplasia and lobular carcinoma in situ. Having had breast cysts, fibrocystic changes (which cause the breasts to feel lumpy), or small growths in the milk ducts called intraductal papillomas does not increase your risk of breast cancer.
- Hormone Replacement Therapy: Using certain hormonal replacement therapies after the beginning of menopause slightly raises your risk of breast cancer. This added risk disappears about three to five years after you stop taking the hormones. The risk is greatest for combination hormone replacement therapy, which uses both estrogen and progestin, as opposed to therapy using estrogen alone.
- Oral Birth Control: Birth control pills raise your risk of breast cancer very slightly, but the increased risk disappears about a decade after you stop taking them. In addition, oral contraceptives may reduce the risk of ovarian cancer.
- Being Overweight or Obese: Excess weight increases your risk of breast cancer. It also increases the possibility that breast cancer returns after treatment, particularly after menopause. The likely reason is 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 higher; obesity is defined as having a BMI of 30 or higher.
- Radiation Exposure: This risk factor is particularly linked to exposure during the first two or three decades of your life. This includes radiation near the breast for the treatment of other cancers such as lymphoma, as well as radiation to treat acne or enlarged thymus glands. The amount of radiation from a mammogram, however, is very small and does not increase your risk by a significant degree.
- Family History or BRCA Mutations: You may be two to three times more likely than the average woman to develop breast cancer if a first-degree relative (your mother, sister, or daughter) has had the disease. Many, but not all, cases of hereditary breast cancer are linked to mutations in the genes BRCA1 and BRCA2. Our understanding of their role and affect on risk continues to evolve.