Objective: To assess the effects of socioeconomic factors on the association between parity and long-term maternal mortality.
Methods: This was a population-based cohort study of mothers with births registered in the Medical Birth Registry of Norway during the period 1967-2009. We estimated age-specific (40-69 years) cardiovascular and noncardiovascular mortality ratios by number of births using Cox proportional hazard models. To assess effect modification by mothers' attained education, we stratified on low (less than 11 years) and high (11 years or greater) educational level. We further evaluated fathers' mortality by number of births using the same analytical approach.
Results: Mothers with low education had higher mortality (cardiovascular: hazard ratio 2.62, 95% confidence interval [CI] 2.34-2.93, noncardiovascular: hazard ratio 1.67, 95% CI 1.62-1.73). Among mothers with low education, cardiovascular mortality increased linearly with each additional birth above one (P trend=.02). In contrast, among mothers with high education, cardiovascular mortality declined with added births (P trend=.045). For noncardiovascular mortality there was no association among mothers with low education, whereas mortality declined with increasing number of births among mothers with high education (P trend<.01). Father's mortality showed similar associations with number of births when stratified on maternal education.
Conclusion: Women's long-term mortality rose with number of births only for cardiovascular causes of death and only among mothers with low education. Partners of women with low education had similar increasing risk with increasing number of births. Maternal educational level is a strong modifier of the association between parity and long-term mortality.
Level of evidence: II.