Medline Plus Breast Cancer Resources: “Breast cancer affects one in eight women during their lives. Breast cancer kills more women in the United States than any cancer except lung cancer. No one knows why some women get breast cancer, but there are a number of risk factors. Risks that you cannot change include

• Age – the chance of getting breast cancer rises as a woman gets older
• Genes – there are two genes, BRCA1 and BRCA2, that greatly increase the risk. Women who have family members with breast or ovarian cancer may wish to be tested.
• Personal factors – beginning periods before age 12 or going through menopause after age 55

Other risks include being overweight, using hormone replacement therapy (also called menopausal hormone therapy), taking birth control pills, drinking alcohol, not having children or having your first child after age 35 or having dense breasts.”