P-202 Comparative analysis of one-step ultrafast versus multi-step conventional warming protocols: a retrospective study of 2,548 single frozen embryo transfers

Study question Does the one-step ultrafast warming protocol impact blastocyst survival, quality, and clinical pregnancy outcomes compared to conventional warming in single frozen embryo transfer (FET) cycles? Summary answer One-step ultrafast warming demonstrated comparable blastocyst quality, blastocyst survival rates, and clinical pregnancy outcomes to multi-step conventional warming. What is known already Conventional blastocyst warming protocols are effective but require multiple steps. This prolonged process increases the risk of procedural variability and may potentially impact clinical outcomes. Recently proposed one-step ultrafast warming protocols simplify the process, reducing handling time and minimizing stress on embryos during warming. Recent studies have demonstrated comparable blastocyst survival and developmental outcomes with ultrafast warming, however comprehensive data on clinical outcomes remains limited. Study design, size, duration This retrospective cohort study evaluated 2,677 single FET cycles performed in a single IVF center between January 2023 and November 2024. Blastocysts were warmed using Kitazato® media, applying either ultrafast (n = 796) or conventional multi-step (n = 1,881) protocols. Both groups followed identical laboratory and clinical procedures, with the choice of warming method determined by the time period. We analyzed biochemical and clinical pregnancy rates following 2,548 single blastocyst transfers (ultrafast: n = 719; conventional: n = 1,829). Participants/materials, setting, methods Blastocysts were warmed using either the conventional (1-min thawing solution, TS; 3-mins dilution solution, DS; 5-mins washing solution, WS; transfer to culture media) or ultrafast warming protocol (1-min TS, hold in WS; transfer to culture media). Blastocyst survival rates and clinical outcomes were compared using univariate analyses (Mann-Whitney U test and Pearson’s Chi-squared test) and logistic regression models, adjusting for key clinical variables. P-values <0.05 were considered significant. Main results and the role of chance Mean maternal age (±SD) was similar between groups (ultrafast: 40.5 (±5.1) years; conventional: 40.7 (±5.2) years). Post-warming survival rates were comparable in both univariate (98.6% ultrafast, n = 785/796 vs. 97.7% conventional, n = 1838/1881, p = 0.171) and adjusted analyses (OR = 1.66, [95%CI: 0.85-3.24], p = 0.138). The proportion of top-quality (Gardner grades AA, AB, BA, or BB) blastocysts were also similar (p = 0.701) between the ultrafast (57.0%, n = 454/796) and conventional group (56.1%, n = 1056/1881). Biochemical pregnancy rates were higher in the ultrafast group in both univariate (ultrafast: 50.0% vs. conventional: 45.4%, p = 0.032) and adjusted analyses (OR = 1.22, [95%CI: 1.02-1.45], p = 0.026). Nevertheless, this did not translate to improved clinical pregnancy rates, which remained similar between the ultrafast (39.9%) and conventional (37.7%) groups in both univariate (p = 0.311) and adjusted analyses (OR = 1.10, [95%CI: 0.92-1.32], p = 0.282). Stratified analyses confirmed that these findings were consistent across day 5 and day 6 blastocyst FETs and between autologous and oocyte donation cycles, reinforcing the generalizability of the results. Limitations, reasons for caution The retrospective, single-center design and sequential implementation of protocols may limit the scope of the findings. Incomplete follow-up data for ongoing pregnancy outcomes in the ultrafast group prevented robust analysis of late pregnancy outcomes. Multicenter, prospective studies are needed to confirm these findings. Wider implications of the findings Ultrafast warming offers a safe and time-efficient alternative to conventional methods, demonstrating equivalent clinical outcomes. This approach offers a streamlined alternative to conventional methods without compromising clinical success. Widespread adoption may enhance patient care by optimizing IVF laboratory efficiency. Trial registration number No