Optimal Timing to Warm Day 5 Vitrified Blastocysts for Frozen Embryo Transfer

OBJECTIVE: To evaluate the differences in pregnancy outcomes after frozen embryo transfer (FET) when day 5 (D5) vitrified blastocysts are warmed on the day prior versus the day of transfer. STUDY DESIGN: In July 2016 our laboratory implemented a protocol change wherein D5 vitrified blastocysts were warmed on the morning of transfer rather than the day prior to transfer. In this retrospective cohort study, transfer cycles before and after the protocol change were compared. The primary outcome measure was clinical pregnancy rate (CPR). Secondary outcome measures included positive pregnancy test (PPT), implantation rate (IMPR), live birth rate (LBR), and spontaneous abortion rate (SABR). RESULTS: In total, 147 FET cycles were analyzed. When all blastocyst transfers before and after the protocol change were compared, the CPR did not differ, nor did PPT, IMPR, LBR, and SABR. The adjusted odds ratio for CPR of D5 blastocyst transfers was 1.30 (95% confidence interval 0.67-3.53, p value 0.30) when comparing embryos warmed the day of to those the day prior to transfer. CONCLUSION: Our findings suggest that pregnancy outcomes are unaffected by day of warming vitrified blastocysts for transfer. Thus, there can be flexibility with the timing of blastocyst warming in the embryology laboratory.