risk factors

Last reviewed 08/2021


The clearest risks for invasive breast cancer are:

  • female sex
  • the presence of preinvasive cancer:
    • lobular carcinoma in situ
    • ductal carcinoma in situ
  • previous breast cancer

Other risk factors (classified as increased risk above normal) include:

  • family history of breast cancer - the risk of breast cancer in a woman has been quantified with respect to the number of affected first degree relatives and by the age of affected first degree relative (1)

    Table 1: Relative risk for breast cancer by number of affected first degree female relatives

    Number of first degree relativesRelative risk for women < 50 years (99% CI)Relative risk for women >= 50 years(CI)
    12.14 (1.92 to 2.38)1.65 (1.53 to 1.78)
    23.84 (2.37 to 6.22)2.61 (2.03 to 3.34)
    312.05 (1.70 to 85.16)2.65 (1.29 to 5.46)

    Table 2: Relative risk for breast cancer by age of affected first degree relative

    Age at time of diagnosis in first degree relativesRelative risk for women < 50 years (99% CI)Relative risk for women >= 50 years (CI)
    < 40 years13.5 (3.4 to 53.9) 3.9 (1.8 to 8.6)
    >= 40 years7.8 (2.4 to 25.0)2.6 (1.8 to 3.7)

  • age - peak incidence 45-75 years but any age postmenarche >> 4x
  • country of residence - high in West > 4x e.g. UK, low in East e.g. Japan
  • previous breast cancer > 4x
  • irradiation of chest - shows a linear dose-response relationship 2-4x
  • social class (I vs. V) 2-4x
  • race - more common in Caucasians < 2x
  • previous ovarian or endometrial cancer < 2x
  • early menarche or late menopause < 2x
  • nulliparity or older than 30 years before first child < 2x
  • hormonal supplementation < 2x
  • obesity - oestrogen synthesis in adipose tissue
  • alcohol consumption

In the male, Klinefelter's syndrome is a risk factor for breast cancer.


  • smoking and breast cancer risk
    • most studies on active cigarette smoking show no association with breast cancer (2)
      • some investigators, however, have argued that most of these studies have failed to consider that both nonsmokers and smokers are exposed to environmental tobacco smoke (ETS), which has potentially masked any effects of active smoking on breast cancer
    • a US observational study concluded that regular ETS exposure is causally related to breast cancer diagnosed in younger, primarily premenopausal women and that the association is not likely explained by bias or confounding (3)


  1. Collaborative Group on Hormonal Factors in Breast Cancer. Familial breast cancer: collaborative reanalysis of individual data from 52 epidemiological studies including 58,209 women with breast cancer and 101,986 without the disease. Lancet 2001;358:1389-99.
  2. Gammon MD et al. Environmental tobacco smoke and breast cancer incidence. Environmental Research 2004; 96 (2):76-185
  3. Miller MD et al. The association between exposure to environmental tobacco smoke and breast cancer: a review by the California Environmental Protection Agency. Prev Med. 2007 Feb;44(2):93-106