Breast cancer is the development of abnormal cells in the breast. These cells are very different from normal, healthy cells. These cells begin to grow out of control and make more cells that grow into tumors, or growths, and can spread to other parts of the body.

Breast cancer develops over time, starting with one tiny, abnormal cell. In most cases this takes a long time, but sometimes the type of cancer grows very fast and the tumor grows and spreads quickly.

Likelihood of having breast cancer

Breast cancer is the most common cancer that women may have to face in their lifetime (other than skin cancer). It can develop at any age, but it is much more likely after age 40. The chance of having breast cancer increases as women get older. Some women – because of certain factors – may have a greater chance of developing breast cancer than other women.

These factors include:

• A personal history of breast cancer
• Inherited changes (or mutations) in breast cancer-related genes (called BRCA1 and BRCA2 genes)
• Previous radiation treatments to the chest
• Two or more close relatives with breast or ovarian cancer
• A relative (mother, sister, grandmother, or aunt) on either side of the family with breast cancer before age 50
• Male relatives with breast cancer

Women who have some of these factors should ask their doctors if they should have an MRI along with their mammograms and clinical breast exams each year. For more information, see our document called Breast Cancer: Early Detection.

Some factors may increase the chance of having breast cancer by only a small amount, such as:

• Beginning your menstrual periods at an early age (also called early menarche)
• Going through menopause at a late age
• Having no children
• Having your first pregnancy after age 30
• Gaining weight as an adult
• Excessive use of alcohol

But most breast cancers occur in women who have none of these risk factors, other than getting older. This means it's important that all women try to find breast cancer early through routine screening mammograms, regular clinical breast exams, and watching for any breast changes.

Rumors about breast cancer risk factors

People with fears about breast cancer sometimes start untrue rumors about what causes breast cancer. These rumors can be hurtful and frightening to others.

For example, some Internet rumors say that antiperspirants and underwire bras can increase a woman's risk of developing breast cancer. There is no experimental or clinical evidence to support either of these claims. Antiperspirants do not contain cancer-causing substances and do not block such substances from getting out of the body. We also know that injuries to the breast do not cause cancer, and that breast cancer is not something a woman gets or catches, like the flu.

If you hear claims about new causes of breast cancer, talk to your doctor before changing your lifestyle or personal habits. The American Cancer Society also has up-to-date information on cancer research and recent findings. You can get this information by calling 1-800-227-2345 or visiting www.cancer.org.