Breast Cancer

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Causes, Risk Factors, and Prevention TOPICS

What are the risk factors for breast cancer?

Most women who have one or more breast cancer risk factors never develop breast cancer, while many women with breast cancer have no known risk factors (other than being a woman and growing older). Even when a woman with risk factors develops breast cancer, it’s hard to know just how much these factors might have contributed.

Some risk factors, like a person's age or race, can't be changed. Other risk factors are linked to cancer-causing factors in the environment or to personal behaviors, such as smoking, drinking, and diet. Some factors affect risk more than others, and your risk for breast cancer can change over time, due to things like aging or lifestyle.

Breast cancer risk factors you cannot change

The main risk factors for breast cancer are things you cannot change: being a woman, getting older, and having certain gene changes. These make your risk of breast cancer higher. But having a risk factor, or even many, does not mean that you are sure to get the disease.

Being a woman

Simply being a woman is the main risk factor for breast cancer. Men can have breast cancer, too, but this disease is about 100 times more common in women than in men. This might be because men have less of the female hormones estrogen and progesterone, which can promote breast cancer cell growth.

Getting older

As you get older, your risk of breast cancer goes up. Most invasive breast cancers (those that have spread from where they started) are found in women age 55 and older.

Certain inherited genes

About 5% to 10% of breast cancer cases are thought to be hereditary, meaning that they result directly from gene defects (called mutations) passed on from a parent.

BRCA1 and BRCA2: The most common cause of hereditary breast cancer is an inherited mutation in the BRCA1 and BRCA2 genes. In normal cells, these genes help prevent cancer by making proteins that help keep the cells from growing abnormally. Mutated versions of these genes cannot stop abnormal growth, and that can lead to cancer.

  • If you have inherited a mutated copy of either gene from a parent, you have a higher risk of breast cancer.
  • In some families with BRCA1 mutations the lifetime risk of breast cancer is as high as 80%, but on average this risk seems to be in the range of 55% to 65%. For BRCA2 mutations the risk is lower, around 45%.
  • Breast cancers linked to these mutations are more often found in younger women and more often in both breasts than cancers not linked to these mutations. Women with these inherited mutations also have a higher risk of developing other cancers, mainly ovarian cancer.
  • In the United States, BRCA mutations are more common in Jewish people of Ashkenazi (Eastern Europe) origin than in other racial and ethnic groups, but anyone can have them.

Changes in other genes: Other gene mutations can also lead to inherited breast cancers. These gene mutations are much less common and most of them do not increase the risk of breast cancer as much as the BRCA genes. They are seldom causes of inherited breast cancer.

  • ATM: The ATM gene normally helps repair damaged DNA. Inheriting 2 abnormal copies of this gene causes the disease ataxia-telangiectasia. Inheriting one abnormal copy of this gene has been linked to a high rate of breast cancer in some families.
  • TP53: The TP53 gene gives instructions for making a protein called p53 that helps stop the growth of abnormal cells. Inherited mutations of this gene cause Li-Fraumeni syndrome. People with this syndrome have an increased risk of breast cancer, as well as some other cancers such as leukemia, brain tumors, and sarcomas (cancers of bones or connective tissue). This mutation is a rare cause of breast cancer.
  • CHEK2: Li-Fraumeni syndrome can also be caused by inherited mutations in the CHEK2 gene. A CHEK2 mutation, even when it doesn’t cause this syndrome, can increase breast cancer risk about 2-fold.
  • PTEN: The PTEN gene normally helps regulate cell growth. Inherited mutations in this gene cause Cowden syndrome, a rare disorder that puts people at higher risk for both non-cancer and cancer tumors in the breasts, as well as growths in the digestive tract, thyroid, uterus, and ovaries. Defects in this gene can also cause a different syndrome called Bannayan-Riley-Ruvalcaba syndrome that’s not thought to be linked to breast cancer risk. The syndromes caused by mutations in PTEN can be grouped together as PTEN Tumor Hamartoma Syndrome.
  • CDH1: Inherited mutations in this gene cause hereditary diffuse gastric cancer, a syndrome in which people develop a rare type of stomach cancer. Women with mutations in this gene also have an increased risk of invasive lobular breast cancer.
  • STK11: Defects in this gene can lead to Peutz-Jeghers syndrome. People affected with this disorder have pigmented spots on their lips and in their mouths, polyps in the urinary and gastrointestinal tracts, and a higher risk of many types of cancer, including breast cancer.
  • PALB2: The PALB2 gene makes a protein that interacts with the protein made by the BRCA2 gene. Mutations in this gene can lead to a higher risk of breast cancer. It isn’t yet clear if PALB2 gene mutations also increase the risk for ovarian cancer and male breast cancer.

For more on this, see Family Cancer Syndromes.

Genetic testing: Genetic testing can be done to look for mutations in the BRCA1 and BRCA2 genes (or less commonly in other genes such as PTEN or TP53). While testing can be helpful in some cases, the pros and cons need to be considered carefully.

If you’re thinking about genetic testing, it’s strongly recommended that you first talk to a genetic counselor, nurse, or doctor who can explain and interpret the results of these tests. It’s very important to understand what genetic testing can and can’t tell you, and to carefully weigh the benefits and risks of genetic testing before these tests are done. Testing costs a lot and might not be covered by some health insurance plans.

For more, see Genetic Testing: What You Need to Know.

Having a family history of breast cancer

It’s important to note that most women (about 8 out of 10) who get breast cancer do not have a family history of the disease, but:

  • Women who have close blood relatives with breast cancer have a higher risk of the disease.
  • Having a first-degree relative (mother, sister, or daughter) with breast cancer almost doubles a woman’s risk. Having 2 first-degree relatives increases her risk about 3-fold.
  • Women with a father or brother who have had breast cancer also have a higher risk of breast cancer.

Overall, less than 15% of women with breast cancer have a family member with this disease.

Having a personal history of breast cancer

A woman with cancer in one breast has a higher risk of developing a new cancer in the other breast or in another part of the same breast. (This is different from a recurrence or return of the first cancer.) This risk is even higher for younger women with breast cancer.

Your race and ethnicity

Overall, white women are slightly more likely to develop breast cancer than African-American women, but African-American women are more likely to die of this cancer. In women under 45 years of age, breast cancer is more common in African-American women. Asian, Hispanic, and Native American women have a lower risk of developing and dying from breast cancer.

Having dense breast tissue

Breasts are made up of fatty tissue, fibrous tissue, and glandular tissue. Someone is said to have dense breasts (on a mammogram) when they have more glandular and fibrous tissue and less fatty tissue. Women with dense breasts on mammogram have a risk of breast cancer that is 1.2 to 2 times that of women with average breast density. Unfortunately, dense breast tissue can also make mammograms less accurate.

A number of factors can affect breast density, such as age, menopausal status, the use of certain drugs (including menopausal hormone therapy), pregnancy, and genetics.

For more on this see Breast Density and Your Mammogram Report.

Certain benign breast conditions

Women diagnosed with certain benign (non-cancer) breast conditions may have a higher risk of breast cancer. Some of these conditions are more closely linked to breast cancer risk than others. Doctors often divide benign breast conditions into 3 general groups, depending on how they affect this risk.

Non-proliferative lesions: These conditions are linked to an overgrowth of breast tissue. They don’t seem to affect breast cancer risk, or if they do, it’s to a very small extent. They include:

  • Fibrosis and/or simple cysts (sometimes called fibrocystic changes or disease)
  • Mild hyperplasia
  • Adenosis (non-sclerosing)
  • Phyllodes tumor (benign)
  • A single papilloma
  • Fat necrosis
  • Duct ectasia
  • Periductal fibrosis
  • Squamous and apocrine metaplasia
  • Epithelial-related calcifications
  • Other benign tumors (lipoma, hamartoma, hemangioma, neurofibroma, adenomyoepthelioma)

Mastitis (infection of the breast) is not a tumor and does not increase the risk of breast cancer.

Proliferative lesions without atypia (cell abnormalities): In these conditions there’s excessive growth of cells in the ducts or lobules of the breast. They seem to raise a woman’s risk of breast cancer slightly. They include:

  • Usual ductal hyperplasia (without atypia)
  • Fibroadenoma
  • Sclerosing adenosis
  • Several papillomas (called papillomatosis)
  • Radial scar

Proliferative lesions with atypia: In these conditions, the cells in the ducts or lobules of the breast tissue grow excessively, and some of them no longer look normal. These types of lesions include:

  • Atypical ductal hyperplasia (ADH)
  • Atypical lobular hyperplasia (ALH)

Breast cancer risk is raised in women with these changes. If a woman has a family history of breast cancer and either hyperplasia or atypical hyperplasia, she has an even higher risk of breast cancer.

For more on these conditions, see Non-cancerous Breast Conditions.

Lobular carcinoma in situ: In lobular carcinoma in situ (LCIS), cells that look like cancer cells are growing in the lobules of the milk-producing glands of the breast, but they do not grow through the wall of the lobules. LCIS is also called lobular neoplasia. It’s sometimes grouped with ductal carcinoma in situ (DCIS) as a non-invasive breast cancer, but it differs from DCIS in that it doesn’t seem to become invasive cancer if it isn’t treated.

Women with lobular carcinoma in situ (LCIS) have a much higher risk of developing cancer in either breast.

Starting menstruation (periods) before age 12

Women who have had more menstrual cycles because they started menstruating early (before age 12) have a slightly higher risk of breast cancer. The increase in risk may be due to a longer lifetime exposure to the hormones estrogen and progesterone.

Going through menopause after age 55

Women who have had more menstrual cycles because they went through menopause later (after age 55) have a slightly higher risk of breast cancer. The increase in risk may because they have a longer lifetime exposure to the hormones estrogen and progesterone.

Having radiation to your chest

Women who as children or young adults were treated with radiation therapy to the chest for another cancer (such as Hodgkin disease or non-Hodgkin lymphoma) have a significantly higher risk for breast cancer. This varies with the patient’s age when they got radiation. And if you had chemotherapy with the radiation, it might have stopped ovarian hormone production for some time, which lowers the risk. The risk of developing breast cancer from chest radiation is highest if you had radiation during adolescence, when your breasts were still developing. Radiation treatment after age 40 does not seem to increase breast cancer risk.

For more information, see Second Cancers Caused by Cancer Treatment.

Exposure to diethylstilbestrol (DES)

From the 1940s through the early 1970s some pregnant women were given an estrogen-like drug called DES because it was thought to lower their chances of losing the baby (miscarriage). These women have a slightly increased risk of developing breast cancer. Women whose mothers took DES during pregnancy may also have a slightly higher risk of breast cancer.

For more on DES see DES Exposure: Questions and Answers.

Lifestyle-related breast cancer risk factors

Certain breast cancer risk factors are related to personal behaviors, such as diet and exercise. Other lifestyle-related risk factors include decisions about having children and taking birth control.

Drinking alcohol

Drinking alcohol is clearly linked to an increased risk of developing breast cancer. The risk increases with the amount of alcohol consumed. Compared with non-drinkers, women who have 1 alcoholic drink a day have a very small increase in risk. Those who have 2 to 5 drinks daily have about 1½ times the risk of women who don’t drink alcohol. Excessive alcohol consumption is known to increase the risk of other cancers, too.

The American Cancer Society recommends that women have no more than 1 alcoholic drink a day. A drink is 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of 80-proof distilled spirits.

Being overweight or obese

Being overweight or obese after menopause increases breast cancer risk. Before menopause your ovaries make most of your estrogen, and fat tissue makes only a small amount. After menopause (when the ovaries stop making estrogen), most of a woman’s estrogen comes from fat tissue. Having more fat tissue after menopause can raise estrogen levels and increase your chance of getting breast cancer. Also, women who are overweight tend to have higher blood insulin levels. Higher insulin levels have been linked to some cancers, including breast cancer.

Still, the link between weight and breast cancer risk is complex. For instance, risk appears to be increased for women who gained weight as an adult, but may not be increased among those who have been overweight since childhood. Also, excess fat in the waist area may affect risk more than the same amount of fat in the hips and thighs. Researchers believe that fat cells in various parts of the body have subtle differences that may explain this.

The American Cancer Society recommends you stay at a healthy weight throughout your life by balancing your food intake with physical activity and avoiding excessive weight gain.

Physical activity

Evidence is growing that physical activity in the form of exercise reduces breast cancer risk. The main question is how much exercise is needed. In one study from the Women’s Health Initiative, as little as 1¼ to 2½ hours per week of brisk walking reduced a woman’s risk by 18%. Walking 10 hours a week reduced the risk a little more.

To reduce your risk of breast cancer, the American Cancer Society recommends that adults get at least 150 minutes of moderate intensity or 75 minutes of vigorous intensity activity each week (or a combination of these), preferably spread throughout the week.

Moderate activity is anything that makes you breathe as hard as you do during a brisk walk. During moderate activities, you’ll notice a slight increase in heart rate and breathing. You should be able to talk, but not sing during the activity. Vigorous activities are performed at a higher intensity. They cause an increased heart rate, sweating, and a faster breathing rate. Activities that improve strength and flexibility, such as weight lifting, stretching, or yoga, are also beneficial.

Having children

Women who have not had children or who had their first child after age 30 have a slightly higher breast cancer risk overall. Having many pregnancies and becoming pregnant at an early age reduces breast cancer risk overall. Still, the effect of pregnancy is different for different types of breast cancer. For a certain type of breast cancer known as triple-negative, pregnancy seems to increase risk.

Birth control

Oral contraceptives: Studies have found that women using oral contraceptives (birth control pills) have a slightly higher risk of breast cancer than women who have never used them. Once the pills are stopped, this risk seems to go back to normal over time. Women who stopped using oral contraceptives more than 10 years ago do not appear to have any increased breast cancer risk. When thinking about using oral contraceptives, women should discuss their other risk factors for breast cancer with their health care provider.

Birth control shot: Depo-Provera is an injectable form of progesterone that’s given once every 3 months as birth control. A few studies have looked at the effect of birth control shots on breast cancer risk. Women currently using birth control shots seem to have an increase in breast cancer risk, but it appears that there is no increased risk in women 5 years after they stop getting the shots.

Hormone therapy after menopause

Hormone therapy with estrogen (often combined with progesterone) has been used for many years to help relieve symptoms of menopause and help prevent osteoporosis (thinning of the bones). This treatment goes by many names, such as post-menopausal hormone therapy (PHT), hormone replacement therapy (HRT), and menopausal hormone therapy (MHT).

There are 2 main types of hormone therapy. For women who still have a uterus (womb), doctors generally prescribe estrogen and progesterone (known as combined hormone therapy or HT). Progesterone is needed because estrogen alone can increase the risk of cancer of the uterus. For women who’ve had a hysterectomy (those who no longer have a uterus), estrogen alone can be used. This is known as estrogen replacement therapy (ERT) or just estrogen therapy (ET).

Combined hormone therapy (HT): Use of combined hormone therapy after menopause increases the risk of breast cancer. It may also increase the chances of dying from breast cancer. This increase in risk can be seen with as little as 2 years of use. Combined HT also increases the likelihood that the cancer may be found at a more advanced stage. (This means it’s already spread from the place it started when it’s found.)

The increased risk from combined HT appears to apply only to current and recent users. A woman’s breast cancer risk seems to return to that of the general population within 5 years of stopping treatment.

Bioidentical hormone therapy: The word bioidentical is sometimes used to describe versions of estrogen and progesterone with the same chemical structure as those found naturally in people. The use of these hormones has been marketed as a safe way to treat the symptoms of menopause. It’s important to realize that because there aren’t many studies comparing “bioidentical” or “natural” hormones to synthetic versions of hormones, there’s no proof that they’re safer or more effective. More studies are needed to know for sure. The use of these bioidentical hormones should be thought to have the same health risks as any other type of hormone therapy.

Estrogen therapy (ET): The use of estrogen alone after menopause does not seem to increase the risk of breast cancer much, if at all. But when used long term (for more than 10 years), ET has been found to increase the risk of ovarian and breast cancer in some studies.

At this time there are few strong reasons to use post-menopausal hormone therapy (either combined HT or ET), other than possibly for the short-term relief of menopausal symptoms. Along with the increased risk of breast cancer, combined HT also appears to increase the risk of heart disease, blood clots, and strokes. It does lower the risk of colorectal cancer and osteoporosis, but this must be weighed against the possible harms, especially since there are other ways to prevent and treat osteoporosis and screening can sometimes prevent colon cancer. ET does not seem to increase breast cancer risk, but it does increase the risk of stroke.

The decision to use HT should be made by a woman and her doctor after weighing the possible risks and benefits (including the severity of her menopausal symptoms), and considering her other risk factors for heart disease, breast cancer, and osteoporosis. If they decide she should try HT for symptoms of menopause, it’s usually best to use it at the lowest dose that works for her and for as short a time as possible.


Some studies suggest that breastfeeding may slightly lower breast cancer risk, especially if it’s continued for 1½ to 2 years. But this has been hard to study, especially in countries like the United States, where breastfeeding for this long is uncommon.

The explanation for this possible effect may be that breastfeeding reduces a woman’s total number of lifetime menstrual cycles (the same as starting menstrual periods at a later age or going through early menopause).

Factors with unclear effect on breast cancer risk

There are some things that might be risk factors for breast cancer, but the research is not yet clear about whether there really is a link. They include things like tobacco smoke and working at night.

Diet and vitamins

Many studies have been done looking for a link between certain diets and breast cancer risk, but so far the results have been conflicting. Results of some studies have shown that diet may play a role, while others showed no evidence that diet influences breast cancer risk.

Studies looking at vitamin levels have had inconsistent results. And some studies have found that higher levels of certain nutrients increased the risk for breast cancer in women. So far, no study has shown that taking vitamins reduces breast cancer risk. This is not to say that there’s no point in eating a healthy diet. A diet low in fat, low in red meat and processed meat, and high in fruits and vegetables can have other health benefits.

Many studies of women in the United States have not linked breast cancer risk to fat in the diet. Still, studies have found that breast cancer is less common in countries where the typical diet is low in total fat, low in polyunsaturated fat, and low in saturated fat. Researchers are still not sure how to explain this. It may be at least partly due to the effect of diet on body weight (see below). Also, studies comparing diet and breast cancer risk in different countries are complicated by other differences (such as activity level, intake of other nutrients, and genetic factors) that might also affect breast cancer risk.

More research is needed to better understand the effect of the types of fat eaten on breast cancer risk. It’s clear that calories do count, and fat is a major source of calories. High-fat diets can lead to being overweight or obese, which is a known breast cancer risk factor. A diet high in fat is also a risk factor for some other types of cancer. And intake of certain types of fat is clearly related to higher risk of heart disease.

Chemicals in the environment

A great deal of research has been reported and more is being done to understand possible environmental influences on breast cancer risk.

Compounds in the environment that have estrogen-like properties are of special interest. For example, substances found in some plastics, certain cosmetics and personal care products, pesticides, and PCBs (polychlorinated biphenyls) seem to have such properties. In theory, these could affect breast cancer risk.

This issue raises a great deal of public concern, but at this time research does not show a clear link between breast cancer risk and exposure to these substances. Studying such effects in humans is hard to do. More research is needed to better define the possible health effects of these substances and others like them.

Tobacco smoke

For a long time, studies showed no link between cigarette smoking and breast cancer. But in recent years, more studies have shown that heavy smoking over a long-time is linked to a higher risk of breast cancer. In some studies, the risk was highest in certain groups, such as women who started smoking before they had their first child. The 2014 US Surgeon General’s report on smoking concluded that there is “suggestive but not sufficient” evidence that smoking increases the risk of breast cancer.

Researchers are also looking at whether secondhand smoke increases the risk of breast cancer. Both mainstream and secondhand smoke contain chemicals that, in high concentrations, cause breast cancer in rodents. Studies have shown that chemicals in tobacco smoke reach breast tissue and are found in breast milk of rodents.

In human studies, the evidence on secondhand smoke and breast cancer risk is not clear, at least in part because the link between smoking and breast cancer is also not clear. One reason for this might be that tobacco smoke may have different effects on breast cancer risk in smokers compared with those who are just exposed to secondhand smoke.

A report from the California Environmental Protection Agency in 2005 concluded that the evidence about secondhand smoke and breast cancer is “consistent with a causal association” in younger, mainly pre-menopausal women. The 2014 US Surgeon General’s report concluded that there is “suggestive but not sufficient” evidence of a link at this point. In any case, this possible link to breast cancer is yet another reason to avoid secondhand smoke.

Night work

Some studies have suggested that women who work at night, such as nurses on a night shift, might have an increased risk of breast cancer. This is a fairly recent finding, and more studies are looking at this. Some researchers think the effect may be due to changes in levels of melatonin, a hormone that’s affected by the body’s exposure to light, but other hormones are also being studied.

Disproven or controversial breast cancer risk factors

There are many factors that research has shown are not linked to breast cancer. You may see information online or hear about these disproven or controversial risk factors, but it is important to learn the facts.


Internet and e-mail rumors have suggested that chemicals in underarm antiperspirants are absorbed through the skin, interfere with lymph circulation, and cause toxins to build up in the breast, eventually leading to breast cancer.

Based on the available evidence (including what we know about how the body works), there is little if any reason to believe that antiperspirants increase the risk of breast cancer. For more information, read Antiperspirants and Breast Cancer Risk.


Internet and e-mail rumors and at least one book have suggested that bras cause breast cancer by obstructing lymph flow. There is no good scientific or clinical basis for this claim, and a recent study of more than 1,500 women found no association between wearing a bra and breast cancer risk.

Induced abortion

Several studies have provided very strong data that neither induced abortions nor spontaneous abortions (miscarriages) have an overall effect on the risk of breast cancer. For more detailed information, read Is Abortion Linked to Breast Cancer?

Breast implants

Several studies have found that breast implants do not increase the risk of breast cancer, although silicone breast implants can cause scar tissue to form in the breast. Implants make breast tissue harder to see on standard mammograms, but additional x-ray pictures called implant displacement views can be used to examine the breast tissue more completely.

Breast implants might be linked to a rare type of lymphoma called anaplastic large cell lymphoma. This lymphoma has rarely been found in the breast tissue around the implants. So far, though, there are too few cases to know if the risk of this lymphoma is really higher in women with implants.

Last Medical Review: 06/01/2016
Last Revised: 09/13/2016