The cause of most breast cancers (including ductal carcinoma) is unknown, although some tend to occur in families. Breast cancer is a common type of cancer among women in the United States, Canada and Europe. However, in Africa and Asia, the prevalence of breast cancer is much lower.

• Age. Breast cancer occurs more often in women over 50 and is less common in women before menopause.
  
  
• Family history. Having a mother or sister with either breast or ovarian cancer increases a woman’s risk of breast cancer.
  
  
• Genetic predisposition. Research continues to reveal numerous genetic mutations associated with many types of breast cancer. The best known are the BRCA1 or BRCA2 breast cancer genes. A woman with BRCA1 or BRCA2 genes has an 80 percent chance of developing breast cancer in her lifetime, according to the American Cancer Society.
  
  
• Ashkenazi Jewish heritage. This segment of the population has a significant prevalence of breast cancer.
  
  
• Previous history of breast cancer or benign breast tumors. This includes being diagnosed with breast cancer, and having biopsies for tumors that turn out to be benign (noncancerous).
  
  
• Race. Breast cancer is more common in white women than in women of other races.
  
  
• Radiation to the chest area. Women with Hodgkin’s lymphoma who are treated with radiation to the chest before age 30 have an increased risk of developing breast cancer.
  
  
• Hormone use. Until recently, hormone replacement therapy (HRT) was commonly prescribed to women to reduce the symptoms associated with menopause. Recent studies have shown strong evidence that HRT or or other extended exposure to estrogen or progesterone can increase a woman’s risk of breast cancer.
  
  
• Breast density. Women with breasts that are less fatty (e.g., older women) have an increased risk of breast cancer.
  
  
• Reproductive history. Women who give birth before the age of 30 have a lower risk of developing breast cancer in their lifetime, compared to women who have children later in life or who never have children at all.
  
• Menstrual history. Women who started their menstrual periods early (before age 12) or went through menopause late (after age 55) have an increased risk of breast cancer.
  
  
• Having taken diethylstilbestrol (DES). A synthetic estrogen widely prescribed from the early 1940s through the early 1970s for pregnant women, DES is believed to slightly increase a woman’s risk of breast cancer.
  
  
• Being obese after menopause. In postmenopausal women, estrogen is primarily produced by fat tissue. As a result, postmenopausal women who are significantly overweight have more estrogen, which increases their risk of breast cancer.
  
  
• Sedentary lifestyle. Women who are physically inactive have a higher rate of breast cancer, possibly due to a sedentary lifestyle leading to a person becoming overweight or obese (an independent risk factor for breast cancer in postmenopausal women).
  
  
• Use of alcohol. Several studies have found that increased consumption of alcohol raises the risk of breast cancer.
  
  
• Smoking. Smoking significantly increases the risk of developing breast cancer, particularly for those with a family history of the disease.