Ethnic Heterogeneity in Reproductive Risk Factors for Breast Cancer, With a Focus on Asian Populations: A Meta-analysis

This study aimed to examine differences in the association between reproductive factors and breast cancer (BC) risk across ethnic groups, particularly Asians and non-Asians, and to explore temporal trends through meta-analysis. The study focused on epidemiologic research published up to August 31, 2022, examining reproductive factors related to BC risk and family history. All effect sizes were calculated using a random-effect model. The protective effect of the higher number of childbirths against BC was stronger in Asians than in Europeans or Americans (childbirths ≥ 2 vs. 1; Asians, relative risk [RR]: 0.66, 95% CI: 0.59-0.74; Europeans, RR: 0.89, 95% CI: 0.86-0.92; Americans, RR: 0.91, 95% CI: 0.87-0.96). Similarly, the effect of high parity was more pronounced in Asians than in Americans and Europeans (Asians, RR: 0.72, 95% CI: 0.58-0.89; Europeans, RR: 0.81, 95% CI: 0.74-0.88; Americans, RR: 0.84, 95% CI: 0.76-0.92). In contrast, no significant differences among populations were found in BC risks associated with combined hormone replacement therapy use. While the association between family history and BC risk appeared to differ by ethnicity, no temporal change was observed (< 2010, RR: 1.58, 95% CI: 1.40-1.78; ≥ 2010, RR: 1.57, 95% CI: 1.46-1.67). These results suggest that some reproductive factors associated with BC differ across ethnicities and time trends, perhaps due to the prevalence of reproductive factors and the baseline hazard of BC.