Like all cancer, breast cancer is caused by genetic mutations that allow cells to divide and grow in a disorganized way. Normal breast cells divide in a regular, ordered fashion, forming new cells that are exact copies to replace old ones. Certain genes in each cell are responsible for telling cells when to divide and when to stop dividing. Other genes identify and fix problems in DNA that is copied incorrectly, or cause cells with bad DNA to self-destruct rather than keep multiplying. If a genetic mutation causes one or more of these genes to turn off in some cells, the cells can divide at a faster rate without regulation or order, becoming more and more mutated. When these disordered cells begin to invade nearby tissues or break off and migrate to other locations, they are called cancer.

Since genetic mutations cause breast cancer, risk factors for breast cancer include anything that can encourage mutations. Breast cancer is not the result of one genetic mutation alone; it requires many different genetic mutations for cancer to develop. Some genetic mutations are inherited from parents, while others can be caused by factors in the environment such as diet, hormone levels, and exposure to radiation. Scientists believe that in most women, breast cancer develops as a combination of inherited and environmental factors. It is still unclear why some women develop breast cancer and some women don’t.

Risk Factors for Breast Cancer

It is important to note that while science is good at finding correlations, or apparent relationships, between factors and disease, correlation does not prove that the factor causes the disease. Many risk factors for breast cancer have been identified and are being studied for their role in the development of the disease.

Age is a factor in breast cancer risk. A woman’s risk for developing breast cancer increases with age. Only 5 percent of breast cancers are diagnosed in women under 40. Breast cancer is most prevalent in women over 70.

Any woman in the population has a 12.5 percent (one in eight) chance of developing invasive breast cancer in her lifetime. Hereditary and environmental factors may raise or lower that risk.

Hereditary Factors

Between 5 and 10 percent of breast cancers are thought to be influenced by genes inherited in families. If women in your family have had breast or ovarian cancers, your risk for breast cancer is increased. Many genetic variants have been identified as being linked with a higher risk for breast cancer. Mutant variants of the BRCA1 and BRCA2 genes raise the risk for developing cancer, but not every woman with these variant genes will get cancer. About 50 percent of women with one of these mutant genes will develop breast cancer before age 70, while only 7 percent of those who do not have the mutant genes will get breast cancer. Some women with a high risk of breast cancer due to inherited factors choose to undergo preventative treatments such as hormone blocking medications or surgery.

Ethnicity seems to influence breast cancer predisposition and sometimes the type of breast cancer women get. Women with Ashkenazi Jewish backgrounds are more likely to carry BRCA1 or BRCA2 mutations. In the U.S., women with European, non-Hispanic backgrounds have the highest rates of breast cancer overall. Women of African descent have higher rates of breast cancer between ages 40 and 50 and also the highest rate of mortality from breast cancer, at least partly due to the tendency to receive diagnosis later, when cancer is more advanced. Inflammatory breast cancer is more common in women of African descent than those of European descent. (Read more about breast cancer types.) Women of Korean descent have the lowest rates of breast cancer, while women of Chinese background are the least likely to die of breast cancer. It is difficult for researchers to separate how much of these differences are due to genes and how much to socioeconomic and cultural factors.

Environmental Factors

Researchers have identified a wide array of environmental factors linked to the development of breast cancer.

Longer exposure to female sex hormones such as estrogen and progesterone has a strong influence over the risk for developing breast cancer. Women with the longest period of fertility – those who begin menstruating before age 12 or enter menopause after age 55 – have the longest exposure to hormones, and subsequently a higher risk for breast cancer. Women who have never been pregnant and those who had their first child after age 30 similarly have increased risk – pregnancy and breastfeeding give the body a break from estrogen and progesterone. Use of hormone replacement therapy after menopause also raises the risk for developing breast cancer, especially at higher doses and for longer durations. Hormonal birth control (“the pill”) seems to cause a slight increase in breast cancer risk, depending on other risk factors.

Learn more about hormone replacement therapy and breast cancer risk.

Smoking cigarettes has been proven in multiple studies to significantly increase the risk for developing breast cancer, especially in women with a family history of breast cancer. Even second-hand smoke may raise the risk for some women to develop breast cancer. Women who continue to smoke after receiving a breast cancer diagnosis have a higher mortality rate than those who quit and those who never smoked.

There is evidence that women who drink alcohol (one or more drinks per day) are 11 percent more likely to develop breast cancer.

Women with higher socioeconomic status – defined as higher income and education – have a higher risk for breast cancer than those of lower socioeconomic status. It is theorized that higher socioeconomic status is not a risk factor itself, but raises the likelihood of other risk factors. Higher socioeconomic status makes it more likely that a women will delay having children, have fewer children, drink alcohol, and use hormonal birth control (“the pill”).

Several studies have investigated specific viruses for links with breast cancer. Some scientists believe that one or more common viruses such as the Epstein-Barr virus (the cause of mononucleosis, also called “mono” or “kissing disease”) and human papilloma virus (HPV, the cause of genital warts and cervical cancer) may cause genetic changes in some people, paving the way for the development of breast cancer.

Obesity has differing effects on breast cancer risk before and after menopause. Premenopausal women have a 20 to 40 percent lower risk for developing breast cancer if they are overweight or obese. However, in postmenopausal women, overweight or obesity – especially when extra weight is carried in the upper body – increases the risk for breast cancer by 30 to 60 percent.

What Does Not Cause Breast Cancer?

Many substances and experiences were thought at one time to increase the risk for developing breast cancer. The following have been thoroughly studied as potential risk factors, and no link with breast cancer has ever been proven:

• Deodorants and antiperspirants
• Wearing an underwire bra, or any other type of bra
• Injury or trauma to the breast
• Cell phones
• Caffeine
• Microwave ovens
• Contact with a person who has cancer
• Breast implants
• Having an abortion

Condition Guide

• Breast Cancer – An Overview
• Breast Cancer: Stage and Grade
• Breast Cancer Symptoms
• Breast Cancer – The Path to Diagnosis
• Breast Cancer Treatment
• Side Effects of Breast Cancer Treatments
• Types of Breast Cancer