A risk factor is anything that affects your chance of getting a disease, such as cancer. But 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).

There are different kinds of risk factors, some that can’t be changed, and some that are lifestyle-related,  such as smoking, drinking, and diet. To read in more detail about both risk factors you cannot change and lifestyle-related risk factors, see below.

Risk Factors You Cannot Change

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

Aging
Your risk of developing breast cancer increases as you get older. About 1 out of 8 invasive breast cancers are found in women younger than 45, while about 2 of 3 invasive breast cancers are found in women age 55 or older.

Genetic risk factors
About 5% to 10% of breast cancer cases are thought to be hereditary, resulting directly from gene defects (called mutations) inherited 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 keep the cells from growing abnormally. If you have inherited a mutated copy of either gene from a parent, you have a high risk of developing breast cancer during your lifetime. The risk may be as high as 80% for members of some families with BRCA mutations.

These cancers tend to occur in younger women and more often affect both breasts than cancers in women who are not born with one of these gene mutations. Women with these inherited mutations also have an increased risk for developing other cancers, particularly ovarian cancer.  In the United States BRCA mutations are found most often in Jewish women of Ashkenazi (Eastern Europe) origin, but they can occur in any racial or ethnic group. Genetic testing can be done to look for mutations in the BRCA1 and BRCA2 genes (or some other genes linked to breast cancer risk). Although testing may be helpful in some situations, the pros and cons need to be considered carefully. For more information, see genetic counseling.

Family history of breast cancer
Breast cancer risk is somewhat higher among women whose close blood relatives have this disease. Having one first-degree relative (mother, sister, or daughter) with breast cancer approximately doubles a woman’s risk. Having two first-degree relatives increases her risk about three-fold.  Altogether, however, less than 15% of women with breast cancer have a family member with this disease. Most (over 85%) of women who get breast cancer do not have a family history of this disease.

Personal history of breast cancer
A woman with cancer in one breast has a three-to four-fold increased risk of developing a new cancer in the other breast or in another part of the same breast. This is different from a recurrence (return) of the first cancer.

Race and ethnicity
Overall, white women are slightly more likely to develop breast cancer than are African-American women, but African-American women are more likely to die of this cancer. However, 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.

Dense breast tissue
Women with denser breast tissue (as seen on a mammogram) have more glandular tissue and less fatty tissue, and have a higher risk of breast cancer. Unfortunately, dense breast tissue can also make it harder for doctors to spot problems on mammograms.

Benign breast conditions
Women diagnosed with certain benign breast conditions may have an increased risk of breast cancer. Some of these conditions are more closely linked to breast cancer risk than others. For more information on these conditions, see Non-cancerous Breast Conditions.

Cells that Look like Cancer Cells

In some women, there are cells that look like cancer cells growing in the lobules of the milk-producing glands of the breast, but they do not grow through the wall of the lobules. This is known as lobular carcinoma in situ (LCIS). Women with this condition have a 7- to 11-fold increased risk of developing invasive cancer in either breast. For this reason, women with LCIS should make sure they have regular mammograms and doctor visits.

Menstrual periods
Women who have had more menstrual cycles because they started menstruating at an early age (before age 12) and/or went through menopause at a later age (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.

Previous chest radiation
Women who, as children or young adults, had radiation therapy to the chest area as treatment for another cancer (such as Hodgkin disease or non-Hodgkin lymphoma) have a significantly increased risk for breast cancer. This varies with the patient’s age when they had radiation. If chemotherapy was also given, it may have stopped ovarian hormone production for some time, lowering the risk. The risk of developing breast cancer from chest radiation is highest if the radiation was given during adolescence, when the breasts were still developing. Radiation treatment after age 40 does not seem to increase breast cancer risk.

Lifestyle-Related Factors 

Having children
Women who have had no children or who had their first child after age 30 have a slightly higher breast cancer risk. Having many pregnancies and becoming pregnant at a young age reduce breast cancer risk. Pregnancy reduces a woman’s total number of lifetime menstrual cycles, which may be the reason for this effect.

Breast-feeding
Some studies suggest that breast-feeding may slightly lower breast cancer risk, especially if breast-feeding is continued for 1½ to 2 years. But this has been a difficult area to study, especially in countries such as the United States, where breast-feeding for this long is uncommon. One explanation for this possible effect may be that breast-feeding reduces a woman’s total number of lifetime menstrual cycles (similar to starting menstrual periods at a later age or going through early menopause).

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 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 drink no alcohol. Excessive alcohol use is also known to increase the risk of developing several other types of cancer.

Being overweight or obese
Being overweight or obese has been found to increase breast cancer risk, especially for women after menopause. Also, women who are overweight tend to have higher blood insulin levels. Higher insulin levels have also been linked to some cancers, including breast cancer. But the connection between weight and breast cancer risk is complex. For example, the 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.

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 (WHI) as little as 1.25 to 2.5 hours per week of brisk walking reduced a woman’s risk by 18%. Walking 10 hours a week reduced the risk a little more.

Recent oral contraceptive use
Studies have found that women using oral contraceptives (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.

Hormone therapy after menopause
Hormone therapy with estrogen (often with progesterone) has been used for many years to help relieve symptoms of menopause and to help prevent osteoporosis (thinning of the bones). There are 2 main types of hormone therapy. For women who still have a uterus (womb), doctors generally prescribe both estrogen and progesterone (known as combined hormone therapy or HT). For women who no longer have a uterus (those who’ve had a hysterectomy), estrogen alone can be prescribed. This is commonly known as estrogen replacement therapy (ERT) or just estrogen therapy (ET).

Combined hormone therapy - Using combined hormone therapy after menopause increases the risk of getting breast cancer. It may also increase the chances of dying from breast cancer. This increase in risk can be seen with as little as two years of use. Combined HT also increases the likelihood that the cancer may be found at a more advanced stage.
The increased risk from combined hormone therapy 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 five years of stopping combined treatment.

Estrogen Therapy - The use of estrogen alone after menopause does not appear to increase the risk of developing breast cancer. In fact, some research has suggested that women who have previously had their uterus removed and who take estrogen actually have a lower risk of breast cancer. Women taking estrogen seem to have more problems with strokes and other blood clots, though. Also, when used long term (for more than 10 years), ET has been found to increase the risk of ovarian cancer in some studies.

Some information provided by The American Cancer Society.