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What Are the Risk Factors for Breast Cancer?

| October 1, 2015


A look at lifestyle choices and other possible concerns

A risk factor is anything that affects your chance of getting a disease, such as cancer. Different cancers have different risk factors. For example, exposing skin to strong sunlight is a risk factor for skin cancer. Smoking is a risk factor for cancers of the lung, mouth, larynx, bladder, kidney, and several other organs.

But risk factors don’t tell us everything. Having a risk factor, or even several, does not mean that you will get the disease. Most women who have one or more breast cancer risk factors never develop the disease, while many women with breast cancer have no apparent risk factors (other than being a woman and growing older).

Some risk factors, like a person’s age or race or gender, can’t be changed. Simply being a woman is the main risk factor for developing breast cancer — it is about 100 times more common among women than men. And the risk of developing breast cancer increases as you age. About one out of eight invasive breast cancers are found in women under 45, while about two of three invasive breast cancers are found in women age 55 or older.

Then there are genetic risk factors. About 5 to 10 percent of breast cancer cases are thought to be hereditary, meaning that they result directly from gene defects (called mutations) inherited from a parent. 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 keep the cells from growing abnormally. If you have inherited a mutated copy of either gene, you have a high risk of developing breast cancer in your lifetime.

Other risk factors are linked to cancer-causing factors in the environment. Still others are related to personal behaviors, such as smoking, drinking, and diet. Some factors influence risk more than others.

Here is an overview of breast cancer risk factors related to lifestyle choices, as well as those factors that are considered unclear or controversial, courtesy of the American Cancer Society.


Having Children

Women who have had no 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 a young age reduce breast cancer risk overall.

Birth Control

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

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 to help prevent osteoporosis (thinning of the bones). Earlier studies suggested it might have other health benefits as well, but these benefits have not been found in more recent, better-designed studies. This treatment goes by many names: post-menopausal hormone therapy (PHT), hormone replacement therapy (HRT), and menopausal hormone therapy (MHT).

There are two main types of hormone therapy. For women who still have their uterus, doctors generally prescribe both estrogen and progesterone (known as combined hormone therapy or HT). Proges­terone is needed because estrogen alone can increase the risk of uterine cancer. For women who’ve had a hysterectomy, estrogen alone can be prescribed. This is known as estrogen replacement therapy (ERT) or just estrogen therapy (ET).

Studies have shown that using combined hor­mone therapy after menopause inc­reases the risk of getting breast cancer. It may also increase the chances of dying from breast cancer. The use of estrogen alone after menopause does not appear to increase the risk of breast cancer.


Some studies suggest that breastfeeding may slightly lower breast cancer risk, especially if it is continued for 1½ to two years. But this has been a difficult area to study, especially in countries such as the U.S. where breastfeeding for that long is uncommon. One explanation for this possible effect may be that breastfeeding reduces a woman’s total number of lifetime menstrual cycles. Women who have had more menstrual cycles because they started having periods early (before age 12) and/or went through menopause late (after age 55) 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.➝

Drinking Alcohol

The use of 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 consume one alcoholic drink a day have a very small increase in risk. Those who have two to five drinks daily have about 1½ times the risk of women who don’t drink alcohol. Excessive alcohol consumption is also known to increase the risk of developing several other types of cancer.

Being Overweight or Obese

Being overweight or obese after menopause increases breast cancer risk. Before menopause, your ovaries produce most of your estrogen, and fat tissue produces a small amount of estrogen. 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 increase your chance of getting breast cancer by raising estrogen levels. Also, women who are overweight tend to have higher blood insulin levels, which have also been linked to some cancers, including breast cancer.

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 1.25 to 2.5 hours per week of brisk walking reduced a woman’s risk by 18 percent. Walking 10 hours a week reduced the risk a little more.


Diet and Vitamin Intake

Many studies have looked for a link between what women eat and breast cancer risk, but so far the results have been conflicting. Some studies have indicated that diet may play a role, while others found no evidence that diet influences breast cancer risk. For example, a recent study found a higher risk of breast cancer in women who ate more red meat.

Studies have also looked at vitamin levels, again with inconsistent results. Some studies actually found an increased risk of breast cancer in women with higher levels of certain nutrients. So far, no study has shown that taking vitamins reduces breast cancer risk. This is not to say that there is 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 might have other health benefits.

Most 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. But many studies of women in the U.S. have not linked breast cancer risk to dietary fat intake. Researchers are still not sure how to explain this apparent disagreement. It may be at least partly due to the effect of diet on body weight. Also, studies comparing diet and breast cancer risk in different countries are complicated by other differences (like activity level, intake of other nutrients, and genetic factors) that might also affect breast cancer risk.

More research is needed to understand the effect of the types of fat eaten on breast cancer risk. But it is 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 breast cancer risk factor. A diet high in fat has also been shown to influence the risk of developing several other types of cancer, and the intake of certain types of fat is clearly related to heart disease risk.

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 (such as DDE), and PCBs (polychlorinated biphenyls) seem to have such properties. These could in theory affect breast cancer risk.

This issue understandably invokes 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. Unfortunately, studying such effects in humans is difficult. More research is needed to better define the possible health effects of these and similar substances.

Tobacco Smoke

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

Night Work

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


As with any topic, Internet and e-mail rumors and misinterpretation of studies lead to the spreading of false information about the breast cancer risks of various factors. Based on the available evidence, little if any reason exists to believe that an increased risk of breast cancer is due to antiperspirants, bras, abortion and miscarriage, and breast implants.

Breast implants may be linked to a rare type of lymphoma called anaplastic large cell lymphoma that has rarely been found in the breast tissue around the implants. So far, though, too few cases exist to know if the risk of this lymphoma is really higher in women who have implants.

For more details on any of these factors, visit the American Cancer Society website, cancer.org/cancer/breastcancer/detailed-guide/breast-cancer-risk-factors.

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