The exact causes of breast cancer remain uncertain. Changes or mutations (inherited or spontaneous) allow breast cancer to develop. BRAC1 and BRAC2, two genes linked to breast cancer, are mutated in 5 to 10% of cases.

Risk factors for developing breast cancer:

Gender and Age:

Women are 100 times more likely to develop breast cancer than men, and the risk increases with age.

Genetics:

A heightened risk for developing breast cancer can be inherited (passed through genes) – having an immediate relative (mother, sister, or daughter) with the disease doubles the chance of getting breast cancer. Chest irradiation, which can cause genetic mutations, increases the risk.

Estrogen:

Elevated estrogen exposure occurring from early onset of menstrual periods (prior to age 12) or late menopause (after age 55) increases the chance of developing breast cancer. First full term pregnancy after the age of 30 as well as never becoming pregnant also increase the risk of developing breast cancer. Alternatively, breast-feeding for longer than one year, first menses after age 15, and premenopausal obesity are all associated with lower estrogen levels and a reduced risk of developing breast cancer.

Breast Abnormalities:

The development of benign breast disease increases the risk for development of breast cancer slightly.

Hormone Replacement Therapy (HRT):

The Women’s Health Initiative clinical trial showed a 26% increase in breast cancer in women who took combined estrogen and progestin (trade name Prempro) [Women's Health Initiative HRT Study]. Another large clinical study conducted in England found an increased risk of developing breast cancer following estrogen only HRT treatment (trade name Premarin) [Million Women Study]. Five years after discontinuation of HRT, the risk for developing breast cancer returns to the background level.

Alcohol:

Drinking one alcoholic beverage a day is associated with a slight risk for breast cancer, while drinking two to five alcoholic beverages per day increases the risk by 1.5-fold.