What Causes Breast Cancer?

No one knows exactly what causes breast cancer, but we do know that certain risk factors – the things that increase a person's chance of getting a disease – are linked to breast cancer. Risk factors change depending on the type of cancer.

There are a number of risk factors, both controllable and uncontrollable, which may increase the chances of developing breast cancer. For instance, the risk factors associated with diet can be controlled, but risk factors such as a person's age or family history can't be changed.

While all women are at risk for breast cancer, the factors listed below are associated with an increased chance of developing the disease.

Major Risk Factors

Gender
Being female is the main risk factor for breast cancer.

Age
The chance of getting breast cancer increases as a woman gets older.

Genetics
Studies show that between five percent and 10 percent of breast cancers appear to be linked to mutations in certain genes (specifically the BRCA1 and BRCA2 genes). If a woman has inherited a mutated gene from either parent, she is more likely to develop breast cancer. More than half of women with inherited mutations will develop breast cancer by the age of 70.

Strong family history
The risk is higher among women whose close blood relatives have the disease. Having a mother, sister or daughter with breast cancer almost doubles a woman's risk, particularly if the relative was diagnosed before age 50. A family history of ovarian cancer also increases your risk of developing breast cancer.

Previous breast cancer
Having had cancer in one breast increases the risk of having it in the other.

Earlier radiation treatment
Women who have had chest area radiation treatment have a significantly increased risk of breast cancer (e.g. mantle radiation for Hodgkin's lymphoma).

A biopsy showing atypical hyperplasia
A previous diagnosis of this condition indicates a moderately increased risk. See the description of hyperplasia for details.

Minor risk factors

Reproductive factors
The risk seems to be higher among women who have not had children and in women who had their first baby after the age of 30.

Menstrual history
Women who began having periods early (before 12 years of age) or who went through menopause later than average also have a small increased risk.

Obesity
Obesity is associated with an increased risk of post-menopausal breast cancer.

Alcohol consumption
At one drink a day, the risk is very small. Those who have two to five drinks daily have about 1.5 times the risk of women who don't drink.

History
A history of previous benign breast biopsies, other than atypical hyperplasia.

Dense breast tissue
If your breast tissue appears dense on a mammogram your risk is slightly higher.

Birth control pills
Taking “the pill” can slightly in increase your risk of breast cancer.

Hormone therapy
Long term use (greater than five years) of combined estrogen and progestin therapy slightly increases breast cancer risk.