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Breast Cancer Causes and Risk Factors

A senior woman sitting in between an older woman and a younger woman on a sofa.

A family history of breast cancer in a first-degree relative (mother, daughter, or sister) increases your risk of breast cancer.

Credit: iStock

Breast cancer is caused by certain changes in how breast cells function, especially how they grow and divide into new cells. A risk factor is anything that increases the chance of getting a disease. Some risk factors for breast cancer, like drinking alcohol, can be changed. However, risk factors also include things people cannot change, like your genetics, getting older, and your family history.

Factors that can increase the risk of breast cancer

There are many risk factors for breast cancer, but most do not directly cause cancer. Instead, they increase the chance of DNA damage in cells that may lead to breast cancer. Learn more about how cancer develops at What Is Cancer?  

Having one or more of these risk factors does not mean that you will get breast cancer and some people who get breast cancer do not have known risk factors. The most important risk factor for breast cancer, besides female biological sex, is increasing age. 

Personal health history and breast conditions

The following personal factors and breast conditions have been linked to a higher risk of breast cancer:

  • Having had breast cancer in the past.
  • Having had ductal carcinoma in situ (DCIS) in the past.
  • Having been exposed to high levels of radiation in the past, particularly at a young age, such as during treatment for childhood cancer.
  • Having a history of certain kinds of breast changes. Learn more about precancerous breast conditions.
  • Having dense breasts, a common condition that not only increases the risk of breast cancer but also makes it more difficult to detect breast cancer on mammograms. Learn more about dense breasts.
  • Having excess body weight, especially after menopause.
  • Taking combination (estrogen plus progestin) hormone replacement therapy for symptoms of menopause. Learn more about Menopausal Hormone Therapy and Cancer.
  • Having been exposed to DES (a synthetic estrogen) before birth (in utero) or having taken it during pregnancy. Learn more about cancer risks linked to DES exposure.

Reproductive history

If your reproductive history leads to a longer exposure to natural estrogen, it can increase your risk of developing breast cancer. Reproductive factors that increase the length of time a woman's breast tissue is exposed to estrogen include:

  • having a longer menstrual history (i.e., having early menarche, later menopause)
  • being at an older age at first birth
  • never having carried a pregnancy to term
  • never having breastfed (among women who have given birth)

Genetics and family history

The following genetics and family history factors have been linked to a higher risk of breast cancer:

Genetic counselors and other specially trained health professionals can help people with breast cancer or with a family history of breast cancer understand:

  • the likelihood that they have a genetic risk factor for breast cancer based on their personal history and their family medical history
  • their options for genetic testing for changes in the BRCA1, BRCA2, and other genes that increase the risk of breast cancer
  • the risks and benefits of learning genetic information
  • how to cope with their genetic testing results
  • how to discuss the results with family members

Learn more about genetic counseling and genetic testing to assess your risk of breast cancer and other cancers at Genetic Testing for Inherited Cancer Risk. 

Behaviors

The following behaviors have been linked to a higher risk of breast cancer:

Race

The incidence of breast cancer varies with race. In the United States, White women have the highest incidence of breast cancer, whereas Black women have the highest death rate. 

Potential risk factors that require more study

Some studies have shown that women who are current or recent users of hormonal birth control pills (also called oral hormonal contraceptives) may have a slight increase in breast cancer risk. Other studies have not shown an increased risk of breast cancer in women using birth control pills. Learn more about Oral Contraceptives and Cancer Risk.

Scientists are also studying whether exposure to certain chemicals in the environment may increase a person's risk of breast cancer. Studies of this kind can be difficult to conduct and interpret for many reasons, making it hard to know which chemicals, if any, may increase the risk of breast cancer.

Understanding your risk of breast cancer

Everyone with breast tissue has some risk of breast cancer, but certain risk factors may increase your risk. 

On average, women have a 1 in 8 (or 13 in 100) chance of developing breast cancer during their lifetime. If you have certain risk factors—for instance, you got your menstrual period at an early age, you were older when your first child was born, or you have a first-degree relative with breast cancer—you may be at higher risk. To help you understand your own risk, your doctor can use a risk calculator, such as NCI's Breast Cancer Risk Assessment Tool, which estimates people's risk of breast cancer based on these and several other clinical, reproductive, and medical history factors.

Three senior women going for a walk with one woman touching the shoulder of another woman.
Breast Cancer Prevention

Learn about behaviors and strategies that can reduce your breast cancer risk.

There are some risk factors that, on their own, confer a very high risk of breast cancer—60 in 100 or more. These include certain harmful genetic changes in genes such as BRCA1, BRCA2, PALB2, and PTEN; a strong family history of breast cancer (with multiple relatives diagnosed); and having had radiation to the chest before age 30. 

Women who may be at increased risk of breast cancer can discuss possible screening or preventive measures with their doctors.

It is important to keep in mind that, even with the best available scientific information, doctors cannot precisely estimate an individual's risk. All estimates are subject to uncertainty. For example, someone might have a personal factor or exposure that influences their risk but that isn’t included in the risk calculator. Also, scientific evidence can change over time, making earlier calculations less accurate. Estimates of risk should be considered best guesses based on the knowledge we have now.

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