In 1940, the lifetime risk of a woman developing breast cancer was 5%, or one in 20. In 1997, that risk was estimated at 12%, or one in eight. In many cases, it's not known why a woman gets breast cancer. In fact, 70% of all women with breast cancer have no known risk factors.

A risk factor is anything that increases a person's chance of getting a disease. Different cancers have different risk factors.

But having a cancer risk factor, or even several of them, does not necessarily mean that a person will get cancer. Some women with one or more breast cancer risk factors never develop it, while most women with breast cancer have no apparent risk factors.

Significantly higher risk

History of previous cancer in one breast, especially if it occurred before menopause (menopause increases breast cancer risk 3- to 4-fold) increases a woman's risk of developing a new breast cancer unrelated to the first one. This is different than a recurrence of the previous breast cancer.

Moderately higher risk

• Getting older. Your risk for breast cancer increases as you age. About 77% of women diagnosed with breast cancer each year are over age 50, and almost half are age 65 and older. Consider this: In women 40 to 49 years of age, there is a one in 66 risk of developing breast cancer. In the 50 to 59 age group, that risk increases to one in 40.
• Direct family history. Having a mother, sister or daughter ("first degree" relative) who has breast cancer puts you at higher risk for the disease. The risk is even greater if your relative developed breast cancer before menopause and had cancer in both breasts. Having one first-degree relative with breast cancer approximately doubles a woman's risk, and having two first-degree relatives increases her risk 5-fold. Having a male blood relative with breast cancer may also increase a woman's risk of the disease.
• Genetics. Carriers of either of two familial breast cancer genes called BRCA1 or BRCA2 are at higher risk. Ten percent of women are carriers of this gene.
• Breast lesions. These include multiple papillomatosis, atypical ductal hyperplasia, and lobular carcinoma in situ.

Slightly higher risk

• Distant family history. This refers to breast cancer in more distant relatives such as aunts, grandmothers and cousins.
• Large cysts in the breast. Large cysts increase your risk especially if accompanied by early atypical hyperplasia (abnormal cell changes in the breast). A previous breast biopsy result of atypical hyperplasia increases a woman's breast cancer risk by 4 to 5 times.

• Age at childbirth. Having your first child after age 30 or never having children puts you at higher risk.
• Early menstruation. Your risk increases if you got your period before age 12.
• Late menopause. If you begin menopause after age 50, your risk increases.
• Weight. Being overweight (especially in the upper body), with excess caloric and fat intake, increases your risk, especially after menopause.
• Excessive radiation. This is especially true for women were given radiation for postpartum mastitis, received prolonged fluoroscopic X-rays for tuberculosis or who were exposed to a large amount of radiation before age 30 -- usually as treatment for cancers such as lymphoma.
• Other cancer in the family. A family history of cancer of the ovaries, cervix, uterus or colon increases your risk.
• Heritage. Female descendents of Eastern and Central European Jews (Ashkenazi) are at increased risk.
• Alcohol. Use of alcohol is linked to increased risk of developing breast cancer. Compared with nondrinkers, women who consume one alcoholic drink a day have a very small increase in risk, and those who have 2 to 5 drinks daily, have about 1.5 times the risk of women who drink no alcohol. Alcohol is also known to increase the risk of developing cancers of the mouth, throat, and esophagus.

Low risk

• Pregnancy before age 18
• Early onset of menopause
• Surgical removal of the ovaries before age 37

• Fibrocystic breast changes
• Multiple pregnancies
• Coffee or caffeine intake
• Antiperspirants
• Underwire bras
• Abortion
• Breast implants

There are other factors that may affect your risk for breast cancer, but medical research has not yet determined the exact role they play. They include taking birth control pills for many years and taking hormone replacement medicine for 10 or more years after menopause. Also, scientists are still investigating whether breastfeeding, smoking, high-fat diets, lack of exercise and environmental pollution increase breast cancer risk.

No. Women with a family history of breast cancer account for only 5% to 10% of all women with the disease. If you have a relative who has breast cancer, you have an increased risk. Still, three-fourths of women with a family history of breast cancer will not develop the disease.

Even if you don't have a family history of breast cancer, you're still at risk for developing the disease, especially after age 50.