An all-37°C thawing method improves the clinical outcomes of vitrified frozen-thawed embryo transfer: a retrospective study using a case-control matching analysis

Abstract Purpose The purpose of this study is to assess the impact of different temperatures and equilibrium times on the clinical outcomes of FET cycles during the thawing procedure and to select a better thawing method to improve clinical outcomes. Methods This retrospective study included 1,734 FET cycles from January 1, 2020, to January 30, 2022. Embryos vitrified using a Kitazato Vitrification Kit were thawed at 37°C in all steps (the case group, denoted the “all-37°C” group) or at 37°C and then at room temperature (RT; the control group, denoted the “37℃-RT” group), according to the kit instructions. The groups were matched 1:1 to avoid confounding. Results After case-control matching, 366 all-37°C cycles and 366 37℃-RT cycles were included. The baseline characteristics were similar (all P > 0.05) between the two groups after matching. FET of the all-37°C group yielded a higher clinical pregnancy rate (CPR; P = 0.009) and implantation rate (IR; P = 0.019) than FET of the 37℃-RT group. For blastocyst transfers, the CPR (P = 0.019) and IR (P = 0.025) were significantly higher in the all-37°C group than in the 37℃-RT group. For cleavage embryo transfers, the CPR and IR were non-significantly higher in the all-37°C group than in the 37℃-RT group (P > 0.05). Conclusions Thawing vitrified embryos at 37°C in all steps during FET cycles can enhance CPR and IR. Well-designed prospective studies are warranted to further evaluate the efficacy and safety of the all-37°C thawing method.