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 ۵۵۷ FET cycles performed at Milad Infertility Center, Mashhad, Iran, from March ۲۰۲۰ to February ۲۰۲۵. 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 = ۳۶۶), each ۱-mm increase in EMT was associated with higher odds of LBR (adjusted odds ratio [aOR]: ۱.۳۷, ۹۵% confidence interval [CI]: ۱.۱۴-۱.۶۵; p < ۰.۰۰۱); inverse probability weighted sensitivity analysis was similar (aOR: ۱.۳۸, ۹۵% CI: ۱.۱۵-۱.۶۷; p < ۰.۰۰۱). A statistically significant interaction between EMT and maternal age was identified (P-interaction < ۰.۰۰۱). In age-stratified analyses, this positive association was most pronounced in women aged ۳۵-۳۹ yr (aOR: ۳.۳۸, ۹۵% CI: ۱.۸۴-۶.۲۳), but was attenuated and non-significant in women < ۳۵ yr and those ≥ ۴۰ yr. Compared with EMT ۸.۰-۸.۹ mm, EMT < ۷.۰ mm was associated with lower odds of LBR (aOR: ۰.۲۷, ۹۵% CI: ۰.۱۰-۰.۷۷; p = ۰.۰۱۴). 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.

کلیدواژه ها:

Endometrium ، Embryo transfer ، Live birth ، Maternal age. ، ضخامت آندومتر ، انتقال جنین فریز شده ، نرخ تولد زنده ، سن مادر.