A close relative with breast cancer (a mother, sister or grandmother), and having certain breast cancer susceptibility genes only accounts for five to seven percent of breast cancer risk. Therefore, most women who are diagnosed with breast cancer have no family history.
The age when a woman has her first child also influences breast cancer risk. About 30% of breast cancer risk is explained by having a child late in life after 30 years of age, or never having children. Having children early in life causes immature breast cells to change and become resistant to the effects of cancer-causing chemicals.
Other factors associated with an increased risk of breast cancer include certain types of benign breast disease, and exposure of the breast to ionizing radiation.