Left-handedness in relation to breast cancer risk in postmenopausal women

Breast cancer risk may be influenced by intrauterine exposure to steroid ho rmones. We evaluated left-handedness, a marker of intrauterine hormone expo sure, in relation to breast cancer risk in our population-based, case-contr ol study. Case women 50-79 years of age with a first diagnosis of invasive breast cancer were ascertained through statewide cancer registries in Wisco nsin, Massachusetts, and New Hampshire. Control women were identified in ea ch state through lists of licensed drivers (for ages 50-64) and Medicare be neficiaries (for ages 65-79), and selected at random to correspond with the age distribution of case women. Exposure information, including handedness , was obtained through a telephone interview. Our results indicated a modes t association between left handedness and breast cancer risk (OR = 1.42; 95 % CI = 1.10-1.83). The effect of left-handedness was modified by age; we ob served the greatest risk ratio in the oldest age group. Left-handedness was not associated with breast tumor laterality. Our results are consistent wi th the hypothesis that intrauterine hormone exposures play a role in the de velopment of breast cancer.