Association of endometrial thickness with live birth rate: a study using the Society for Assisted Reproductive Technology Clinic Outcome Reporting System

Objective: To assess the relationship between endometrial thickness (EMT) and live birth rates (LBRs) in fresh embryo transfer and frozen embryo transfer (FET) with and without preimplantation genetic testing (PGT). Design: Retrospective cohort study using the Society for Assisted Reproductive Technology Clinic Outcome Reporting System. Subjects: Autologous in vitro fertilization fresh embryo transfer and FET cycles initiated in 2019–2020. Exposure: Endometrial thickness measured in millimeters. Main Outcome Measures: Live birth rate. Results: A total of 244,001 embryo transfer cycles met the inclusion criteria (100,419 FET cycles with PGT, 96,249 FET cycles without PGT, and 47,333 fresh embryo transfer cycles). An increase in EMT was associated with an increase in LBR among all cycle types until a threshold of 9 mm, after which there was minimal increase in LBR. Before 9 mm, each 1-mm increase in EMT was associated with a relative increase in the odds of live birth by 19% for FET with PGT (adjusted odds ratio [aOR], 1.19; 95% confidence interval [CI], 1.66–1.22), 13% for FET without PGT (aOR, 1.13; 95% CI, 1.09–1.16), and 15% for fresh embryo transfer (aOR, 1.15; 95% CI, 1.09–1.20). Conclusion: The LBR increased with an increase in EMT for fresh and frozen transfers with or without PGT until a threshold of 9 mm, beyond which the LBR plateaued. There was no thickness above 9 mm associated with a decrease in LBR.