Linear association between endometrial thickness and live birth in single blastocyst transfers: A dual‐center retrospective cohort study

Abstract Introduction Our objective was to investigate the linear relationship between endometrial thickness (EMT) and live birth rates (LBR) in single vitrified‐warmed blastocyst transfer (SVBT) cycles, and evaluate EMT's independent effect on LBR. Material and Methods A retrospective cohort study analyzed 3375 SVBT cycles conducted at two reproductive centers between June 2016 and December 2022. EMT was stratified into tertiles (<7 mm, 7–8.8 mm, 8.8–10.0 mm, >10.0 mm). Generalized linear models and restricted cubic splines were used to assess the EMT–LBR relationship, adjusting for potential confounders, including maternal age, body mass index, gravidity, parity, and blastocyst quality. Subgroup analyses examined interactions with demographic and clinical variables. Results Significant positive associations were observed between EMT and LBR across all subgroups. For each 1 mm increase in EMT, the likelihood of live birth increased by 7% (OR = 1.07, 95% CI: 1.03–1.11, p = 0.001). The highest improvements in LBR were seen in women with EMT between 8.8 and 10 mm (OR = 2.37), with a plateau effect observed beyond 10 mm. Subgroup analyses confirmed the consistency of this association across clinical subgroups, with no significant interactions with variables such as maternal age, gravidity, and hormonal profiles. The restricted cubic spline analysis further supported a robust linear positive correlation, remaining stable even after adjusting for potential confounders. These findings emphasize the significant role of EMT in predicting live birth outcomes in assisted reproductive technology. Conclusions The study confirms a significant linear association between EMT and live birth in SVBT cycles. Higher EMT is consistently linked to improved LBRs, with the most benefit observed around 10 mm. These findings highlight EMT as a key predictor in assisted reproductive technology and emphasize the need for further research to optimize endometrial preparation strategies.