Age modifies the association between endometrial thickness and live birth after frozen embryo transfer: A retrospective cohort study

Background: The impact of endometrial thickness (EMT) on reproductive outcomes remains a critical consideration in assisted reproductive technology, yet its age-specific influence requires further clarification. Objective: To assess the association between EMT and live birth rate (LBR) in frozen embryo transfer (FET) cycles. Materials and Methods: This retrospective cohort study included 557 FET cycles performed at Milad Infertility Center, Mashhad, Iran, from March 2020 to February 2025. Live birth was analyzed by multivariable logistic regression and clinical pregnancy by modified Poisson regression. EMT was modeled continuously and categorically. Spline models assessed non-linearity, inverse probability weighting assessed sensitivity to missing covariate data, and an EMT-by-age-group interaction was tested. Results: In complete-case adjusted analyses (n = 366), each 1-mm increase in EMT was associated with higher odds of LBR (adjusted odds ratio [aOR]: 1.37, 95% confidence interval [CI]: 1.14–1.65; p < 0.001); inverse probability weighted sensitivity analysis was similar (aOR: 1.38, 95% CI: 1.15–1.67; p < 0.001). A statistically significant interaction between EMT and maternal age was identified (P-interaction < 0.001). In age-stratified analyses, this positive association was most pronounced in women aged 35–39 yr (aOR: 3.38, 95% CI: 1.84–6.23), but was attenuated and non-significant in women < 35 yr and those ≥ 40 yr. Compared with EMT 8.0–8.9 mm, EMT < 7.0 mm was associated with lower odds of LBR (aOR: 0.27, 95% CI: 0.10–0.77; p = 0.014). Conclusion: EMT is a crucial predictor of live birth in FET cycles, but its clinical significance is strongly dependent on maternal age. These findings support an age-stratified approach to counseling and decision-making when EMT is low in FET cycles.