L26/P-773 Endometrial thickness change and live birth in fresh in vitro fertilization cycles: a baseline thickness-dependent effect

Abstract Study question Does endometrial thickness change during stimulation predict live birth, and is this association modified by baseline endometrial thickness at ovarian stimulation start? Summary answer Endometrial thickness change predicts live birth only when baseline thickness at Gn start is ≥ 3.0 mm, indicating a clear effect-modifying role of baseline endometrial status. What is known already Endometrial thickness measured on the day of embryo transfer has been widely studied as a predictor of IVF outcomes, whereas the clinical relevance of endometrial thickness change during ovarian stimulation remains controversial. Prior studies have reported inconsistent associations between endometrial thickness change and pregnancy outcomes, possibly due to failure to account for baseline endometrial conditions. Whether baseline endometrial thickness modifies the effect of subsequent endometrial growth on live birth in fresh IVF cycles, particularly in women of advanced reproductive age, is currently unclear. Study design, size, duration This retrospective cohort study included 1,280 fresh IVF cycles conducted between 2019 and 2024 at a reproductive center of university affiliated hospital. Participants/materials, setting, methods Women aged ≥35 years undergoing fresh embryo transfer with at least one high-quality embryo were included. Multivariable logistic regression was used to evaluate the association between endometrial thickness change (EMT_diff) and live birth, adjusting for relevant clinical covariates. Effect modification by Gn-start endometrial thickness was assessed using interaction terms, Johnson–Neyman analysis, and stratified models based on a 3.0 mm cut-off. Main results and the role of chance In the overall cohort, EMT_diff was independently associated with higher odds of live birth (adjusted OR 1.31, 95% CI 1.10–1.56). A significant interaction was observed between EMT_diff and Gn-start endometrial thickness (P for interaction = 0.014), indicating effect modification. Johnson–Neyman analysis identified a threshold of approximately 3.0 mm, above which the positive association between EMT_diff and live birth became statistically significant. Stratified analyses confirmed these findings. Among women with Gn-start endometrial thickness <3.0 mm (n = 253), EMT_diff was not associated with live birth. In contrast, among women with Gn-start endometrial thickness ≥3.0 mm (n = 1,027), greater EMT_diff was significantly associated with increased live birth rates (adjusted OR 1.31, 95% CI 1.08–1.59). Baseline endometrial thickness itself was not an independent predictor of live birth in the ≥3.0 mm group. These results are unlikely to be explained by chance given the consistency across interaction, threshold, and stratified analyses. Limitations, reasons for caution The retrospective design limits causal inference. Endometrial thickness was measured using routine clinical ultrasound, which may introduce measurement variability. External validation in independent cohorts is warranted. Wider implications of the findings Endometrial growth provides clinically relevant prognostic information only after a minimal baseline thickness is achieved. Incorporating baseline-dependent endometrial thickness change may improve individualized decision-making in fresh IVF cycles. Trial registration number No