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No one knows exactly why a normal breast cell becomes a cancerous one, and there is probably no single cause. It is thought, however, that breast cancer results from a combination of risk factors. These risk factors can be grouped into several categories:

Hereditary risk
It has long been known that women whose mothers or sisters had breast cancer have a higher risk of developing the disease themselves. Recently, it has been discovered that breast cancer can develop when a woman inherits a breast cancer susceptibility gene from one of her parents. The most common of these genes are the BRCA1 and BRCA2 genes. These genes account for about 10 percent of all breast cancer cases and in families that have these genes, the risk of breast cancer can be very high. However, it is important to realize that 85 - 90 percent of breast cancers and NOT hereditary, and that all women need to be screened for breast cancer, even if no one in their families have ever had the disease.

The female hormones estrogen and progesterone are involved in breast cancer formation. For example, it is known that women who start to menstruate at an early age, or who have a late menopause have a higher risk of breast cancer than women who do not. It is also known that women who take hormone replacement therapy after menopause have an increased risk of breast cancer. Many studies have also been done looking at the risk of taking birth control pills. To date, these studies have not demonstrated an increased risk of breast cancer associated with use of these medications.