Age at last birth and risk of developing breast cancer: a meta-analysis

Objectives Results from epidemiologic studies on age at last birth (ALB) and the risk of developing breast cancer were inconsistent. Therefore, we conducted this meta-analysis to evaluate the association between ALB and the risk of developing breast cancer quantitatively. Methods Relevant articles published up to May 2019 were identified by searching systematically in PubMed, Web of Science, China National Knowledge Infrastructure, and Wan Fang Med Online. Pooled relative risks (RRs) with 95% confidence intervals (CIs) were calculated using a random-effects model. Dose-response relationship was assessed by restricted cubic spline model. Results Fourteen studies with 132 133 cases and 1 314 005 participants were eligible for this meta-analysis. The pooled RR (95% CI) of breast cancer for the highest vs. the lowest stratification of ALB was 1.22 (1.04–1.42). In the subgroup analysis, significant positive associations were also found in studies conducted in Europe (RR: 1.21, 95% CI: 1.06–1.38), studies with results adjusted for parity (RR: 1.26, 95% CI: 1.04–1.54), and studies with results adjusted for age at first birth (RR: 1.37, 95% CI: 1.08–1.74). The results of the dose-response analysis indicated that the departure from linearity was NS between ALB and the risk of breast cancer ( P nonlinearity = 0.711), but the linear associations were NS. Conclusion This meta-analysis suggested that ALB was positively associated with the risk of breast cancer. The risk of developing breast cancer increased gradually with the ALB for women. Our findings may have implications for family planning.