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

Purpose The purpose of this study is to assess the impact of different temperatures and incubation 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 1734 FET cycles from January 1, 2020, to January 30, 2022. Embryos vitrified using a KITAZATO Vitrification Kit were thawed at 37 degrees C in all steps (the case group, denoted the "all-37 degrees C" group) or at 37 degrees C and then at room temperature (RT; the control group, denoted the "37 degrees C-RT" group), according to the kit instructions. The groups were matched 1:1 to avoid confounding. Results After case-control matching, 366 all-37 degrees C cycles and 366 37 degrees C-RT cycles were included. The baseline characteristics were similar (all P > 0.05) between the two groups after matching. FET of the all-37 degrees C group yielded a higher clinical pregnancy rate (CPR; P = 0.009) and implantation rate (IR; P = 0.019) than FET of the 37 degrees C-RT group. For blastocyst transfers, the CPR (P = 0.019) and IR (P = 0.025) were significantly higher in the all-37 degrees C group than in the 37 degrees C-RT group. For D3-embryo transfers, the CPR and IR were non-significantly higher in the all-37 degrees C group than in the 37 degrees C-RT group (P > 0.05). Conclusions Thawing vitrified embryos at 37 degrees C in all steps with shortening wash time can enhance CPR and IR in FET cycles. Well-designed prospective studies are warranted to further evaluate the efficacy and safety of the all-37 degrees C thawing method.