The precise cause of breast cancer is unknown; it is most likely due to a combination of genetic and environmental factors. Some women who exhibit many of the risk factors of breast cancer never contract the disease, while some who manifest none of the risk factors may succumb to it.
There does appear to be a link between breast cancer and the hormone estrogen. It's possible that the more that a woman is exposed to estrogen, the greater risk she has of breast cancer at some point. Researchers are investigating events in a woman's life, such as her age of first and last menstruation, her age at childbirth and whether she breastfed or underwent hormone therapy, as these events all affect the estrogen levels in the body.
The two most important risk factors to doctors right now are a woman's family history of breast cancer and her age. A woman with a blood relative who had breast cancer is two to three times more likely to develop the disease herself, while women who are older than 60 have a greater risk than younger women [source: WebMD]. But there are other risk factors as well, which we'll talk about next.