Hamadan University of Medical Sciences Journal of Research in Health Sciences 2228-7795 26 3 2026 08 15 A Systematic Review and Dose-Response Meta-Analysis of the Association Between the Age of First Pregnancy and Breast Cancer Risk e00685 e00685 10.34172/jrhs.13923 EN Jalal Poorolajal https://orcid.org/0000-0002-3758-3006 Fatemeh Shahbazi https://orcid.org/0000-0003-2320-4885 Nazanin Azmi-Naei https://orcid.org/0000-0001-8778-6314 Amin Doosti-Irani Bita Azmi-Naei https://orcid.org/0009-0003-6709-4881 REVIEW 10.34172/jrhs.13923 2026 02 11 2026 02 24 Introduction: Despite numerous investigations, variations in breast cancer (BC) risk across different ages at first pregnancy remain inconclusive. Thus, this meta-analysis aimed to clarify this association and provide a comprehensive evaluation of the evidence. Study Design: A systematic review. Methods: PubMed, Web of Science, and Scopus databases were systematically searched to identify observational studies published up to March 16, 2025, examining BC risk in relation to age at first pregnancy. Statistical analyses were performed using Stata 18 and Review Manager 5.4. Heterogeneity and potential sources were assessed using the I² statistic and meta-regression, respectively. Publication bias was also evaluated with Begg’s and Egger’s tests. Pooled effect sizes were expressed as risk ratios (RR) or odds ratios (OR) with 95% confidence intervals (CI) using a random-effects model. Results: Of 15,586 identified records, 141 studies involving 1,045,983 participants met the inclusion criteria. The pooled RR of BC for age at first pregnancy≥20 years versus<20 years was 1.18 (95% CI: 1.09–1.28), and the pooled OR was 1.31 (95% CI: 1.26–1.37). Moreover, dose-response meta-analysis demonstrated a consistent upward trend, indicating that BC risk increases by 8–12% for every 5-year delay in the first pregnancy. Conclusion: Our findings revealed that BC risk increases progressively with older age at first pregnancy. However, the overall evidence was moderate due to study limitations. Accordingly, further research is warranted to clarify underlying mechanisms and population-specific variations.