Causal Effects of Female Reproductive and Hormonal Factors on Osteoporosis, Bone Mineral Density, and Osteoarthritis: A Two-Sample Mendelian Randomization Study

Purpose: The causal links between female reproductive and hormonal factors and bone diseases remain uncertain, partly due to conflicting observational evidence and potential reverse causation. We used two-sample Mendelian randomization (MR) to clarify these relationships. Methods: Using large-scale genome-wide association studies (GWAS), predominantly from individuals of European ancestry, we analyzed 12 reproductive and hormonal exposures, including reproductive milestones, fertility history, and hormonal interventions, against 7 bone-related outcomes. Genetically predicted causal effects were estimated using inverse-variance weighted (IVW) MR after removing pleiotropic outliers via the MR-PRESSO test. Heterogeneity and pleiotropy were assessed using MR-Egger, Steiger directionality, and leave-one-out analyses. Results: After FDR correction (FDR < 0.05), genetically predicted effects showed that later age at menarche was associated with higher osteoporosis risk (OR: 1.59) and lower bone mineral density (BMD) (OR: 0.88), whereas later age at menopause was associated with reduced osteoporosis risk (OR: 0.74) and, after outlier removal, higher BMD in women aged 45– 60 years (OR: 1.17). Ever use of hormone replacement therapy was also associated with higher mid-life BMD in women aged after outlier correction (OR: 0.71). Longer menstrual cycles were associated with lower BMD (OR: 0.83), and later age at last oral contraceptives (OCP) use showed a modest association with lower BMD (OR: 0.65). Regarding osteoarthritis, later age at first and last live birth was associated with lower risk (OR: 0.81 and 0.75, respectively), and later age at starting OCP was associated with reduced osteoarthritis risk (OR: 0.88). Conclusion: This study provides genetic evidence for the protective role of a longer reproductive lifespan on bone health. Hormonal factors exert distinct causal effects on osteoporosis and osteoarthritis, which is critical for informing targeted prevention strategies. Keywords: Mendelian randomization, osteoporosis, osteoarthritis, reproductive factors, bone mineral density, estrogen