According to the National Cancer Institute, the following risk factors may increase your risk of breast cancer:

• Estrogen (endogenous) - Endogenous estrogen is a hormone made by the body. It helps the body develop and maintain female sex characteristics. Being exposed to estrogen over a long time may increase the risk of breast cancer. Estrogen levels are highest during the years a woman is menstruating. A woman's exposure to estrogen is increased in the following ways:
  • Early menstruation: Beginning to have menstrual periods at age 11 or younger increases the number of years the breast tissue is exposed to estrogen.
  • Late menopause: The more years a woman menstruates, the longer her breast tissue is exposed to estrogen.
• Estrogen (exogenous) - Estrogen that is made in a laboratory is called exogenous estrogen. Taking exogenous estrogen together with progesterone increases the risk of breast cancer. Exogenous estrogen may be taken in the following ways:
  • Hormone replacement therapy: Hormones (estrogen, progesterone, or both) given to postmenopausal women or women who had their ovaries removed, to replace the estrogen no longer produced by the ovaries.
  • Oral contraceptives (the "pill"): The most commonly used oral contraceptive contains estrogen.
• X-rays - Radiation can cause breast cancer. Treatment with radiation during childhood seems to increase breast cancer risk more than treatment as an adult. For example, radiation used to treat Hodgkin disease by age 16, especially radiation to the chest and neck, increases the risk of breast cancer.Radiation therapy to treat cancer in one breast does not appear to increase the risk of developing cancer in the other breast.
• Obesity -Obesity increases the risk of breast cancer in postmenopausal women.
• Alcohol -Drinking alcohol may increase the risk of breast cancer.
• Genetics -Women who have inherited certain mutations in the BRCA1 and BRCA2 genes have a higher risk of breast cancer. BRCA1 and BRCA2 are tumor suppressor genes. If these genes have a mutation and don't work as they should, the risk of breast cancer is increased. Research is being done to identify other high-risk genes.

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