Background: The selection of an optimal thawing and transfer strategy is a critical determinant of success in frozen embryo transfer (FET) cycles. To investigate the optimal FET strategy, this study analyzed the effects of extended culture duration after warming on clinical outcomes following embryo transfer. Methods: We retrospectively analyzed 9981 FET cycles following either an unsuccessful fresh embryo transfer or cycle cancellation. In these cycles, embryos were warmed and transferred on varying days, spanning from Day 2 to Day 5. We compared baseline characteristics, thawing recovery, and embryo development across groups. Additionally, we performed regression analyses to examine the relationship between clinical outcomes and in vitro culture conditions. Results: The post-warming embryo survival rate was 98.60%, with an available embryo rate of 86.88%. For warmed Day 2 embryos, extending culture to Day 3 was significantly associated with higher clinical pregnancy and live birth rates. Similarly, Day 3 warmed embryos cultured to Day 4 or Day 5 were associated with better outcomes compared with Day 3 transfer. However, there was no significant difference between transferring embryos on Day 4 or Day 5 after warming on Day 4. Likewise, no significant differences were observed when all embryos were cultured to Day 5, regardless of the initial warming day. Conclusion: Tailoring FET strategies with an optimized post-warming culture duration is associated with improved clinical outcomes in assisted reproductive technology. Direct transfer of frozen-warmed blastocysts yielded similar success rates to those undergoing extended culture to the blastocyst stage.
Wang et al. (Tue,) studied this question.