Breast cancer develops when genetic mutations (changes) cause cells in the breast to grow and divide uncontrollably. Normally, breast cells grow in an ordered manner, forming new cells to replace the old. Most researchers believe a combination of hereditary and environmental factors leads to changes in genes that make cells grow out of control. This unchecked growth can lead to breast cancer tumors.

Breast cancer research has found evidence of many factors that can raise the risk for breast cancer. Some people may develop breast cancer without having any known risk factors, while others may seem to have a high risk and never develop cancer.

Here's an overview of some of the most studied causes and risk factors for breast cancer. You can control some of these risks (called modifiable risk factors), while others are beyond anyone’s control (nonmodifiable risk factors). It’s important to remember that having any of these risk factors is not a guarantee that you’ll develop breast cancer.

1. Aging

As you age, your risk for breast cancer increases. Most breast cancer cases are diagnosed in people over 50. About 10 percent of breast cancers are diagnosed in people under 45, and the risk continues to increase with age. This makes it important to get regular breast cancer screenings as you get older.

2. Inherited Genetic Mutations Like BRCA1 and BRCA2

Between 5 percent and 10 percent of breast cancers are thought to be influenced by genes passed down in families. One of the most significant inherited risk factors for breast cancer involves inherited mutations in the BRCA genes, known as BRCA1 and BRCA2. The normal versions of these genes help fix damaged DNA and regulate cell growth and division, but gene mutations allow cancer to develop.

Some people may develop breast cancer without having any known risk factors, while others may seem to have a high risk and never develop cancer.

People with BRCA1 or BRCA2 mutations have between a 45 percent and 85 percent lifetime risk of developing breast cancer, significantly higher than the general population’s 12 percent risk. These BRCA mutations can also increase the risk of ovarian cancer.

However, not every person with these mutations will develop cancer. Genetic testing can help identify who’s at risk. Some people with these genetic risk factors choose preventive measures such as increased screening, medication, or surgery.

3. Family History of Breast Cancer

Having a family history of breast cancer can increase your own risk, particularly if a first-degree relative (such as a parent, child, or sibling) has had the disease. The more family members affected, the higher your risk. If you have one first-degree relative with breast cancer, your risk nearly doubles. Genetic testing and counseling may help determine your risk based on your family history.

4. Ethnicity and Genetic Predisposition

Ethnicity can influence breast cancer risk due to genetic and environmental factors. For example, people of Ashkenazi Jewish descent are about five times more likely to carry BRCA1 or BRCA2 mutations, significantly increasing their risk of breast cancer.

According to the National Cancer Institute, in the U.S., white women have the highest overall incidence of breast cancer. Black women tend to be diagnosed with more aggressive types, such as triple-negative breast cancer, and face higher mortality rates.

These disparities can stem from genetic differences, as well as access to health care and cultural factors. No matter your background, it’s important to understand your risks and get recommended screenings.

5. Hormonal Factors

Hormonal exposure plays a role in breast cancer risk. Estrogen and progesterone, two sex hormones, influence breast cancer development, particularly with longer-term exposure. You may have a higher risk for breast cancer if you:

• Begin menstruation early or reach menopause late
• Take certain types of hormone replacement therapy during menopause for many years
• Have your first child after 30
• Have never had a full-term pregnancy

If you’re concerned about risk factors relating to hormone exposure, talk to your doctor about ways you can reduce these risks.

6. Radiation Exposure

Radiation therapy for other cancers, especially treatments involving the chest, increases the risk of developing breast cancer later in life. People who had radiation therapy in the breast or chest area for Hodgkin’s lymphoma before 30, for example, have an elevated risk of developing breast cancer.

It may help lower the risk of developing breast cancer to avoid cigarette smoke, alcohol, and exposure to certain plastics and pesticides.

7. Obesity and Higher Body Weight

Having a higher body weight, particularly after menopause, can raise your risk of breast cancer. Researchers aren’t sure exactly why, but one theory is that fat tissue produces estrogen, which fuels certain types of breast cancer.

Postmenopausal people with a body mass index (BMI) score in the overweight or obese range are 1.2 to 1.4 times as likely to develop breast cancer than those with a BMI score in the healthy range. Maintaining a healthy weight through exercise and a balanced diet can help lower this risk.

Read about foods to eat and foods to avoid if you’re living with breast cancer.

8. Drinking Alcohol

Drinking alcohol has been shown to increase breast cancer risk. The more you drink, the higher your risk. According to the Breast Cancer Research Foundation, women who consume one alcoholic drink per day have a 7 percent to 10 percent higher chance of developing breast cancer compared to those who do not consume alcohol.

Cutting back on alcohol, or avoiding it altogether, may help lower your risk of developing breast cancer.

9. Smoking

Though smoking is best known for increasing the risk of lung cancer, it also raises the likelihood of breast cancer, especially for premenopausal women. Researchers are also studying whether secondhand smoke can increase breast cancer risk, although study results vary. Most health experts recommend avoiding smoking environments when possible.

10. Environmental Exposure to Certain Chemicals

Some chemicals present in the environment, such as pesticides or endocrine-disrupting chemicals like bisphenol-A (BPA) found in plastics, have been studied for their link to breast cancer. While evidence is still emerging, some studies suggest these chemicals may contribute to breast cancer risk. Reducing exposure to these chemicals by avoiding certain plastics and pesticides may help lower your overall cancer risk.

Factors That Do Not Cause Breast Cancer

Despite widespread misconceptions, there’s no evidence that the following factors increase the risk of breast cancer:

• Deodorants and antiperspirants
• Underwire bras, or any other type of bra
• Breast injury or trauma
• Cellphones
• Caffeine
• Microwave ovens
• Contact with a person who has cancer
• An abortion or miscarriage

There have been false claims about some risk factors. Deodorant, cell phones, caffeine, and microwave ovens don’t increase breast cancer risk.

The Bottom Line

Understanding the factors that increase the risk of breast cancer can empower you to make informed health decisions. If you have concerns about your genetic risk or lifestyle factors, talk to your health care provider about cancer prevention, genetic testing, and regular screenings to catch potential issues early.

By maintaining a healthy lifestyle — eating well, exercising, limiting alcohol, and avoiding smoking — you can reduce your risk of developing breast cancer.

You can take action to educate others about breast cancer risk factors, treatment options, and types of breast cancer. Read about seven ways to participate in Breast Cancer Awareness Month.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 73,000 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Are you living with breast cancer? Have any risk factors impacted your diagnosis? Share your experience in the comments below, or start a conversation by posting on your Activities page.