Impact of combining fast vitrification and fast warming protocols on live birth outcomes.

RESEARCH QUESTION: Does the combination of fast vitrification and fast warming protocols, compared with standard vitrification and standard warming protocols, result in differences in pregnancy outcomes following frozen embryo transfer (FET)? DESIGN: This was a retrospective comparative cohort study of unique patients undergoing either natural, medicated or programmed FET cycles of single blastocysts. The first FET following fresh cycles was included for each patient. Three groups were analysed: standard vitrification with standard warming protocol (n = 1025); standard vitrification with fast warming protocol (n = 926); and fast vitrification with fast warming protocol (n = 471). The primary outcome was live birth rate. Secondary outcomes included positive pregnancy test rate, biochemical pregnancy rate, clinical pregnancy rate and miscarriage rate. Logistic regression models were performed, controlling for day of cryopreservation (day 5 or day 6), FET preparation, body mass index and oocyte age. A subanalysis was performed on embryos that underwent preimplantation genetic testing for aneuploidy (PGT-A). RESULTS: Unadjusted analysis showed no difference in live birth rate (42.5% versus 41.8% versus 46.9%; P = 0.167), pregnancy rate (58.8% versus 56.3% versus 62.4%; P = 0.085), biochemical pregnancy rate (8.9% versus 8.1% versus 8.5%; P = 0.827), clinical pregnancy rate (49.6% versus 48.1% versus 53.3%; P = 0.179), and spontaneous abortion rate (6.9% versus 6.0% versus 6.2%; P = 0.704) across the standard vitrification-standard warming, standard vitrification-fast warming, and fast vitrification-fast warming groups, respectively. These findings remained unchanged in the adjusted analysis. A subanalysis of PGT-A embryos showed no difference in live birth rate (45.1% versus 46.2% versus 49.6%; P = 0.472) between the standard vitrification-standard warming (n = 548), standard vitrification-fast warming (n = 424), and fast vitrification-fast warming (n = 266) groups, respectively. CONCLUSIONS: The fast vitrification-fast warming protocol for vitrified and warmed blastocyst-stage embryos is as effective as the standard vitrification-standard warming protocol, demonstrating no negative impact on live birth or secondary outcomes.