The risk of breast cancer is clearly related to hormonal influences. Genetic Factors and Family History of Breast Cancer - Some families appear to have a genetic tendency for breast cancer. The genes p53 and BARD1 also appear to be important. These facts suggest that breast cancer is caused by the growth of genetically damaged cells. Such genetic damage is known to gradually accumulate in the cells of the body over time.
Hormonal influences are important because they encourage cell growth.
High levels of hormones during a woman's reproductive years, especially when they are not interrupted by the hormonal changes of pregnancy, appear to increase the chances that genetically damaged cells will grow and cause cancer. Also, women who have never had children or who had them after the age of 30 have an increased risk. No one knows how long the effects of the pill last after stopping it. Hormone Replacement Therapy - For more than 5 years has been shown to slightly increase the risk of breast cancer and increases with longer use. Physical Characteristics - Obesity is controversial as a risk factor.