Genetic
Family history has long been known to be a risk factor for breast cancer. Both maternal and paternal relatives are important. The risk is highest if the affected relative developed breast cancer at a young age, had cancer in both breasts, or if she is a close relative. First degree relatives, (mother, sister, daughter) are most important in estimating risk. Several second-degree relatives (grandmother, aunt) with breast cancer may also increase risk. Breast cancer in a male increases the risk for all his close female relatives.

Hormonal
Hormonal influences play a role in the development of breast cancer. Women who start the periods at an early age (11 or younger) or experience a late menopause (55 or older) have a slightly higher risk of developing breast cancer. Conversely, being older at the time of the first menstrual period and early menopause tend to protect one from breast cancer. Having a child before age 30 years may provide some protection and having no children may increase the risk of developing breast cancer.

Lifestyle & Dietary
Breast cancer seems to occur more frequently in countries with high dietary intake of fat, and being overweight or obese is a known risk factor for breast cancer, particularly in postmenopausal women. This link is thought to be an environmental influence rather than genetic. For example, Japanese women, at low risk for breast cancer while in Japan, increase their risk of developing breast cancer after coming to the United States.

The use of alcohol is also an established risk factor for the development of breast cancer. The risk increases with the amount of alcohol consumed.

Studies are also showing that regular exercise may actually reduce a woman's risk of developing breast cancer. Studies have not definitively established how much activity is needed for a significant reduction in risk. One study from the Women's Health Initiative (WHI) showed that as little as one and a quarter to two and a half hours per week of brisk walking reduced a woman's breast cancer risk by 18%.

Environmental
Radiation treatment increases the likelihood of developing breast cancer but only after a long delay. For example, women who received radiation therapy to the upper body for treatment of Hodgkin disease before 30 years of age have a significantly higher rate of breast cancer than the general population.

Updated:: 13/03/2019 []