Breastfeeding experience and breast cancer risk among postmenopausal women.

Results of studies of breastfeeding and postmenopausal breast cancer risk have been inconsistent, with many investigators concluding that breastfeeding does not influence risk. We examined whether breastfeeding reduces postmenopausal breast cancer risk as well as the details of this relationship, including possible modification in risk by the age that a woman first breastfed a child and the number of children she breastfed. This population-based case-control study compared 974 women who were residents of Los Angeles County and newly diagnosed with breast cancer to 973 women with no history of breast cancer who were matched to patients by age (within 3 years) and neighborhood of residence. Subjects were parous and postmenopausal. Breast cancer patients were ages 55-64 years at diagnosis. Women who breastfed at least 16 months experienced a reduced odds of breast cancer relative to women who never breastfed (odds ratio, 0.73; 95% confidence interval, 0.52-1.01). Risk decreased as the number of children breastfed increased, but the association was attenuated after accounting for lifetime duration of breastfeeding. Breast cancer risk was 30% lower among women ages 20-24 years at first breastfeeding than women who had never breastfed (odds ratio, 0.69; 95% confidence interval, 0.54-0.88), independent of the effect of age at first birth. This study provides some evidence that the protective effect of breastfeeding persists into the postmenopausal years. The potential for nondifferential error in recall of breastfeeding habits among postmenopausal patients and controls may explain the inconsistent results observed across studies and underscores the need for careful assessment of this relationship.