The parity paradox: does number of children (parity) influence breast cancer mortality across the life course?

While parity (number of births) has been extensively studied in relation to breast cancer risk, its age-specific impact on breast cancer mortality remains unclear. We examined how the relationship between parity and breast cancer mortality rates varies across age groups. This retrospective cohort study was based on 894,608 Israeli women born between 1940 and 1960 and followed for 31 years (1990-2020). Employing an age-stratified approach (30-49, 50-64, and 65-80 years), we used Cox regression to examine the associations between number of children (0, 1-2, and ≥ 3) and breast cancer mortality rates, adjusting for sociodemographic variables and calendar year of entry into each age group. Amongst the 30-49 bracket, those with 1-2 children (hazard ratio [HR] = 1.656; 99% confidence interval [CI] 1.349-2.033) or ≥ 3 children (HR = 1.551; 99% CI 1.271-1.893) had a greater breast cancer mortality risk than childless women. In the 50-64 bracket, these differences disappeared after adjustment (HR = 1.071; 99% CI 0.949-1.209 and HR = 0.935; 99% CI 0.830-1.054, respectively). In the 65-80 bracket, those with 1-2 children continued to exhibit an elevated risk (HR = 1.237; 99% CI 1.045-1.466), no significant difference existing between women with ≥ 3 children and childless women (HR = 0.989; 99% CI 0.834-1.173). An age-dependent relationship thus obtained between parity and breast cancer mortality. These findings highlight the need for age-tailored approaches to breast cancer risk assessment and prevention.