What Causes Breast Cancer? Risk Factors to Know

What Causes Breast Cancer? Risk Factors to Know

What Causes Breast Cancer? Risk Factors to Know

Age and other factors may influence a person's risk of developing breast cancer.

Breast cancer is a complex and often confusing disease. It isn't clear what causes normal cells to grow out of control and multiply.

Researchers do know that certain hormonal, genetic, lifestyle, and environmental risk factors increase the likelihood of breast cancer. But it's also true that people with few or no risk factors still develop breast cancer, and people with many risk factors at play never develop breast cancer.

With that said, here's what we know about what causes breast cancer, plus risk factors that may increase the likelihood of developing it.

What Causes Breast Cancer?

Simply put, breast cancer happens when breast cells grow abnormally. Researchers attribute these cell changes to a complicated interplay of genetic and environmental factors, though an individual breast cancer patient's cause may remain unknown.

As abnormal cells divide, they accumulate into a mass or lump and may also spread (metastasize) into other parts of the body, like the lymph nodes.

Most breast cancers in women and men start in the ducts that produce milk; this is called invasive ductal carcinoma. Other breast cancers start in other breast tissue cells, including glandular tissue called lobules; this is called invasive lobular carcinoma.

It's also important to mention gene mutations here. Researchers estimate that somewhere between 5% and 10% of all breast cancers occur due to inherited gene mutations, which are passed from generation to generation via DNA. And while there are many hereditary gene mutations that increase the risk of breast cancer, the two most well known are BRCA1 and BRCA2.

BRCA1 and BRCA2 are actually tumor suppressor genes that, when functioning as they're supposed to, keep breast cells from growing abnormally. But when these genes have a mutation and don't function as they're meant to, it increases the risk of breast (as well as ovarian) cancer. According to the CDC, one in 500 women carries a mutation in her BRCA1 or BRCA2 genes.

If you're curious about your own hereditary risk of breast and other cancers, talk to your doctor about genetic testing.

Breast Cancer Risk Factors

The risk of breast cancer is much greater in women than men. Women in the U.S. have a one-in- eight chance of developing this type of cancer sometime during their lifetime. By contrast, a man born in the U.S. today has about a one-in-800 chance of being diagnosed with breast cancer during his lifetime.

Risk factors for male breast cancer aren't well understood. While researchers have identified a handful of genetic and environmental factors that may play a role, most men have no known risk factor other than older age. Male breast cancer is diagnosed at age 71, on average.

Much more is known about risk factors in women. That being said, having one (or several) risk factors doesn't destine a woman to a future of breast cancer, just as a lack of risk factors doesn't mean she'll never develop breast cancer.

The following risk factors are associated with the development of breast cancer in women:


Risk of breast cancer goes up as a woman gets older, which is why the American College of Obstetricians and Gynecologists encourages yearly mammograms starting at age 40 for women of average risk.⁹


Females have a much greater risk of developing breast cancer than males.

Personal and family history

A personal history of lobular carcinoma in situ (abnormal cells found in the milk-making glands of the breast) or atypical hyperplasia (non-cancerous changes in breast cells) increases the risk of breast cancer. A history of cancer in one breast also increases the likelihood of developing cancer in the other breast. Additionally, having a family history of breast cancer puts you at greater risk.

Alcohol consumption

The more alcohol a woman drinks, the higher the risk of developing breast cancer.


With any kind of chest-area radiation treatment in childhood or young adulthood, the risk of breast cancer goes up.

Menstruation age

If you started your period before age 12, your risk for breast cancer increases.

Menopause age

If you start menopause after 55, your risk for breast cancer increases.

Childbirth age

Giving birth to a child after the age of 30 ups your risk.


Women who have never been pregnant carry a greater risk of breast cancer than women who have been pregnant one or more times.

Hormone therapy

Hormone therapy for menopause, specifically any type that combines estrogen and progesterone, increases the risk for certain kinds of breast cancer.

Body Mass

A higher body mass index (an estimate of body fat) is linked to a higher risk of breast cancer in postmenopausal women. Research suggests that extra fat cells, which create estrogen, may be responsible for the higher breast cancer risk.¹


Breast cancer occurs when cells mutate and multiply. What sets off those changes isn't always clear. Research suggests that genetic, hormonal, environmental, and lifestyle factors may be at work. Because women are much more likely than men to develop breast cancer, there's a bounty of research on what may cause breast cells to grow out of control. Knowing the risk factors for breast cancer can help women start a conversation with their healthcare providers about preventive measures, including breast cancer screening.


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