Does birth history account for educational differences in breast cancer mortality? A comparison of premenopausal and postmenopausal women in Belgium.

This study investigates the impact of reproductive factors on the association between education and breast cancer mortality in Belgium. The role of reproductive factors has been investigated in several studies, with mixed results. Reproductive factors are either completely or partially responsible for the association between education and breast cancer mortality. The data consist of the 1991 census linked to registration data on cause-specific mortality during the period 1991-1995, including all breast cancer deaths in Belgium during the observation period. The study population includes all women aged 35-79 at time of the census. Age-standardized mortality rates and mortality rate ratios (Poisson regression) are computed for educational groups with and without control for reproductive factors. The population is stratified according to age (women aged 35-49 and 50-79) and according to nulliparity. The relationship between education and breast cancer is significant among postmenopausal women. Breast cancer mortality is higher among the higher educated women. These results are consistent with international findings, the gradient not being negative as in most other causes of death, but positive. Statistical control for parity and age at first birth reduces the association largely. In addition, among nonparous women, differences in breast cancer mortality by education are not consistent and generally not significant. Reproductive factors are largely responsible for the positive association between education and breast cancer mortality among postmenopausal women in Belgium. Among premenopausal women, the relation is not significant, a pattern consistent with international studies.