Risk Factors for Breast Cancer

By OBOS Common Medical Conditions Contributors | October 15, 2011
Last Revised on Apr 23, 2014

Although risk factors for breast cancer have been identified, none is a “big red flag” that shows a direct relationship or cause in the way cigarette smoking is for lung cancer. Most breast cancer occurs in women with no family history or known genetic risk.

In fact, 70 to 80 percent of women with breast cancer have none of the known risk factors besides age. Only 5 to 10 percent of cases are in women with high-risk mutations of the BRCA1 and BRCA2 genes.

Breast cancer incidence and mortality risk vary according to geography, culture, race, ethnicity and socioeconomic status. While white women are more likely to have breast cancer than African-American women, young African-American women are at greater risk for an aggressive form of breast cancer.

Known Risk Factors

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Age: A woman’s risk of developing breast cancer increases with age. Women ages 50-59 have a 1 in 42 chance of being diagnosed with breast cancer, while at age 85, the number jumps to 1 in 8 or 9 women.

The incidence of breast cancer in women younger than 50 is about 2 percent of all breast cancers diagnosed, but the chance of dying from this disease at a younger age is greater than that for women after menopause (who tend more to die from other diseases).

Breast cancer is the leading cause of cancer death in women ages 35-54 years old, because women in that age bracket do not have a high death rate in general.

Personal history of breast cancer: Such history increases one’s risk of developing a new breast cancer in the other breast by about 0.8 to 1 percent a year up to 10 years, and then levels off.

Genetics: A woman with an inherited BRCA1 or BRCA2 mutation or two or more first-degree relatives (a sibling, parent or child) with premenopausal breast cancer has a significantly increased risk of developing both ovarian and breast cancer. (For more information, see BRCA: Genetic Testing and Inherited Risk.)

Reproductive factors: All of these factors are associated with an increased risk: starting menstrual periods early (before age 12); birth of a first child after age 30; no full-term pregnancies; older age (55+) at menopause; not having breastfed for at least six months.

Hormonal factors: Postmenopausal obesity, which is thought to increase one’s estrogen levels; recent oral contraceptive use, with some evidence suggesting that oral contraceptives slightly increase the risk of breast cancer among women under 35 [4]; long-term use of combined hormone therapy; long-term use (more than five to seven years) of estrogen alone; high bone density after reaching menopause (suggesting high estrogen levels).

Other factors: Alcohol consumption (more than one drink per day or five per week); lack of exercise in adolescence as well as adulthood; a breast biopsy showing certain pathologies such as atypical ductal or lobular hyperplasias (the cells lining the milk ducts of the breast or the cells that produce breast milk grow abnormally); history of high-dose radiation exposure to the chest area — often used, for example, in the treatment of Hodgkin’s disease or lymphomas.

Having denser breasts — with more glandular tissue in relation to fatty tissue — is a risk factor. More research is being done to try to figure out why dense breast tissue is a risk factor and what, if anything, can be done for women with dense breasts.

Environmental Contributors to Increased Risk

Today one in eight women will be diagnosed with breast cancer at some point in their lives, up from 1 in 20 in 1940. As we see a rise in unexplained breast cancer cases in the last few decades, there is increasing scientific evidence that involuntary environmental exposures may play a role in the increased incidence of breast cancer.

Since the post–World War II industrial boom, production of synthetic materials has increased approximately 350 times. The use of chemicals in the United States continues to rise by 3.3 percent every year.

Of the more than 84,000 chemicals in commercial use in the U.S. today, more than 90 percent have never been tested for human health effects. Approximately 2,500 new chemicals are submitted to the U.S. Environmental Protection Agency each year for approval.

Industrial chemical products and emissions historically have not routinely been screened for health effects despite widespread exposures.

Non-industrialized countries have lower breast cancer incidence rates than industrialized countries. Migrant women from countries with low incidence rates who move to industrialized nations soon acquire the higher breast cancer risk of the new country. For more information, see “Breast Cancer & the Environment.”

Behaviors Associated With Lower Risk

Even though the causes of most breast cancer are unknown and nothing is guaranteed to prevent cancer, some studies have shown that certain health behaviors are associated with lower risk:

  • Using hormone therapy during perimenopause only if needed and for a limited time. *Excellent evidence
  • Breastfeeding: the longer you nurse, the more protective the effect against premenopausal breast cancer. *Strong evidence
  • Delaying the start of menstruation (to age 12 or later) by avoiding excessive caloric intake and increasing exercise prior to puberty. *Good to strong evidence
  • Getting more than three hours of exercise every week. *Good evidence
  • Limiting alcohol to no more than one drink a day. *Good evidence
  • Limiting postmenopausal weight gain. *Good evidence.
  • Eating more fruits and vegetables. *Evidence showing direct connection is mixed
  • Consuming less saturated and trans fats. *Evidence showing direct connection is mixed
  • Breastfeeding: the longer we nurse, the more protective the effect against premenopausal breast cancer. *Strong evidence
  • Delaying menstruation in girls by avoiding excessive caloric intake and increasing exercise in prepuberty. *Good to strong evidence Girls used to start menstruating on average around age sixteen, a century ago; now the average age is around twelve, with some girls starting younger.
  • Getting more than three hours of exercise every week. *Good evidence
  • Limiting alcohol to no more than one drink a day. *Good evidence
  • Eating more fruits and vegetables. *Evidence for direct connection is mixed
  • Consuming less saturated and trans fats, which are associated with health problems. *Evidence for direct connection is mixed

The content under the heading Environmental Contributors to Increased Risk is excerpted with permission from the fact sheet “What You Should Know About Breast Cancer & the Environment” by Breast Cancer Action, a national, feminist grassroots education and advocacy organization working to end the breast cancer epidemic.