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Maternal age and breast cancer risk.

Maternal age at time of birth was investigated as a risk factor for breast cancer in a study of 1,176 matched case-control pairs. There was no pattern of increasing adjusted relative risk of breast cancer with increasing maternal age, nor was the mean maternal age of cases older than that of controls. Similar negative results were found among the subset of subjects up to 35 years old, a group previously found to show marked maternal age effects. Thus previous reports of an association between breast cancer and advanced maternal age may have been due to chance, extraneous factors, or a misleading reliance on unadjusted mean maternal ages, rather than on relative risks.