Embryological and clinical predictors of implantation and live birth in 3000 single euploid embryo transfer cycles: a retrospective cohort study

PURPOSE: Although single euploid embryo transfer (SEET) improves implantation and live birth (LB) rates, unsuccessful outcomes may still occur despite the transfer of morphologically high-quality euploid embryos. This study aimed to identify post-thaw embryological and clinical factors associated with implantation and LB outcomes. METHODS: This retrospective cohort study included 3000 frozen-thawed SEET cycles performed at a single center. Post-thaw embryological parameters evaluated were inner cell mass (ICM) and trophectoderm (TE) grades, expansion grade, presence of necrotic areas (NA) and excluded/extruded blastomeres (EB), embryo biopsy/freezing/transfer day (Day 5 vs. Day 6), cryo-survival rate (≥90% vs. <90%), and re-expansion status. Clinical variables included maternal age, body mass index (BMI), and number of previous IVF cycles. Outcomes were analyzed with adjustment for endometrial preparation using modified natural cycle (mNC) or hormone replacement cycle (HRC) protocols. Multivariate logistic regression analysis was performed to identify independent predictors of implantation and LB. RESULTS: Higher cryo-survival (≥90%), Day 5 embryo biopsy/freezing/transfer, and post-thaw re-expansion were associated with increased pregnancy rates (p<0.05). Higher embryo quality, particularly A-grade ICM, was independently associated with increased LB rates (p<0.05). The presence of NA and EB in thawed euploid blastocysts was significantly associated with reduced cryo-survival and lower LB outcomes (p<0.05). Maternal age <35 years was associated with a higher likelihood of LB compared with age ≥43 years (OR: 0.291, 95% CI 0.149-0.569, p<0.001). LB rates were significantly higher in mNC-prepared cycles compared with HRC (85.2% vs. 71.8%, p<0.001). Lower BMI and fewer previous IVF cycles were also associated with improved LB outcomes. CONCLUSION: Post-thaw embryological competence and maternal characteristics significantly influence implantation and live birth outcomes, even in euploid embryo transfers. Comprehensive evaluation of thawed embryo morphology combined with individualized endometrial preparation may optimize clinical outcomes following SEET.