Being a woman: Simply being a woman is the main risk factor for breast cancer. Men can have breast cancer, too, but this disease is about 100 times more common in women than in men. This might be because men have less of the female hormones estrogen and progesterone, which can promote breast cancer cell growth.
Getting older: As you get older, your risk of breast cancer goes up. Most invasive breast cancers (those that have spread from where they started) are found in women age 55 and older.
Certain inherited genes:About 5% to 10% of breast cancer cases are thought to be hereditary, meaning that they result directly from gene defects (called mutations) passed on from a parent.
Genes: The most common cause of hereditary breast cancer is an inherited mutation in the BRCA1 and BRCA2 genes. Other gene mutations can also lead to inherited breast cancers. These gene mutations are much less common and most of them do not increase the risk of breast cancer as much as the BRCA genes. They are seldom causes of inherited breast cancer.
Reproductive factors associated with prolonged exposure to endogenous estrogens, such as early menarche, late menopause, late age at first childbirth are among the most important risk factors for breast cancer. Exogenous hormones also exert a higher risk for breast cancer. Oral contraceptive and hormone replacement therapy users are at higher risk than non-users. Breastfeeding has a protective effect.