No one knows the exact causes of breast cancer. Doctors can seldom explain why one woman gets breast cancer and another does not.

Doctors do know that bumping, bruising, or touching the breast does not cause breast cancer. And breast cancer is not contagious. No one can "catch" this disease from another person.

However, research has shown that women with certain risk factors are more likely than others to develop breast cancer. A risk factor is anything that increases a person's chance of developing a disease. Studies have found the following risk factors for breast cancer:

1. The chance of getting breast cancer goes up as a woman gets older.

2. A woman who has had breast cancer in one breast has an increased risk of getting this disease in her other breast.

3. A woman's risk of breast cancer is higher if her mother, sister, or daughter had breast cancer, especially at a young age (before age 40).

4. The older a woman is when she has her first child, the greater her chance of breast cancer. Women who began menstruation (had their first menstrual period) at an early age (before age 12), went through menopause late (after age 55), or never had children also are at an increased risk.

5. Breast cancer occurs more often in white women than Latina, Asian, or African American women.

6. Women who had radiation therapy to the chest (including breasts) before age 30 are at an increased risk of breast cancer. This includes women treated with radiation for Hodgkin's lymphoma. Studies show that the younger a woman was when she received radiation treatment, the higher her risk of breast cancer later in life.

7. Older women who have mostly dense not fatty) tissue on a mammogram(x-ray of the breast) are at increased risk of breast cancer.

8. After menopause, women who are obese have an increased risk of developing breast cancer. Being obese means that the woman has an abnormally high proportion of body fat. Because the body makes some of its estrogen (a hormone) in fatty tissue, obese women are more likely than thin women to have higher levels of estrogen in their bodies. High levels of estrogen may be the reason that obese women have an increased risk of breast cancer. Also, some studies show that gaining weight after menopause increases the risk of breast cancer.

9. Women who are physically inactive throughout life appear to have an increased risk of breast cancer. Being physically active may help to reduce risk by preventing weight gain and obesity.

It is helpful to be aware of risk factors. But it's also important to keep in mind that most of the women who have these risk factors do not get breast cancer.

A woman who thinks she may be at risk of breast cancer should discuss this concern with her health care provider. The health care provider may suggest ways to reduce the risk and can plan an appropriate schedule for checkups.