O-125 Ultra-fast vs. traditional warming: a study on human oocyte and blastocyst survival, in vitro development, and reproductive success

Study question Does Ultra-Fast Warming achieve similar results to the Traditional Warming protocol for human oocytes and embryos? Summary answer Ultra-Fast Warming achieves comparable outcomes to the Traditional Warming protocol while significantly reducing processing time and improving overall IVF efficiency. What is known already Cryopreservation of oocytes and embryos is one of the most widely used techniques in assisted reproduction, reaching its highest efficiency through vitrification. However, traditional protocols are time-consuming for embryologists to perform. Recently, protocols have emerged that significantly reduce vitrification and warming times. Despite this, there is still no evidence using human oocytes and embryos to confirm the effectiveness of these protocol modifications. In this study, we compare the efficiency of the Traditional Warming protocol (TW) used to date with the emerging Ultra-Fast Warming protocol (UFW) in IVF cycles involving human oocytes and embryos. Study design, size, duration In this retrospective study, 688 single-blastocyst cryotransfer (UFW: 317, TW: 371) and 36 ICSI cycles (UFW: 18, TW: 18; 412 MII) with donor oocytes (DO), performed in 2024, were analyzed. Samples were previously vitrified with standard protocol (12’–15’ in equilibration solution, 90’’ in vitrification solution). TW protocol: specimens were incubated 1’ in thawing solution, 3’ in dilution solution, and 6’ in washing solution; UFW protocol: 1’ in thawing solution, then transferred into culture medium. Participants/materials, setting, methods Groups were comparable. For cryotransfer cycles, oocyte age (UFW: 28.6±6.0; TW: 29.4±6.3; p = 0.078), recipient age (UFW: 40.6±5.0; TW: 40.7±4.8; p = 0.883), gamete origin (DO: UFW: 70.4%, TW: 68.8%, p = 0.649; sperm donor: UFW: 35.2%, TW: 41.8%, p = 0.060), blastocyst quality (≥3BB: UFW: 92.6%, TW: 89.9%, p = 0.218), and culture day (Day 5: UFW: 83.3%, TW: 82.0%, p = 0.645) were evaluated. Oocyte donors ages (UFW: 25.7±3.4; TW: 24.9±3.6; p = 0.535) and proportions of donor sperm (UFW: 11/18; TW: 8/18; p = 0.505) were equipollent. Main results and the role of chance For the 317 single transfers performed using the UFW protocol, 324 blastocysts were devitrified, achieving a survival rate of 97.8%, which was comparable to the TW protocol (371/378 = 98.1%; p = 0.793). Once transferred, pregnancy (UFW: 60.9%, TW: 61.2%; p = 0.935), clinical pregnancy (UFW: 53.0%, TW: 52.3%; p = 0.639), and miscarriage rates (UFW: 7.1%, TW: 11.3%; p = 0.161) were statistically equivalent. All pregnancies remain ongoing at the time of analysis. Regarding oocytes, survival rates were higher in cycles using the UFW protocol (91.8±12.9%) compared to TW cycles (85.2±22.4%), though not statistically significant (p = 0.259). Fertilization (UFW: 65.4±24.6%, TW: 70.7±16.9%; p = 0.383), ICSI-induced degeneration (UFW: 8.4±12.9%, TW: 4.8±8.4%; p = 0.379) and blastocyst formation rates (UFW: 47.4±25.8%, TW: 43.7±26.3%; p = 0.682) were equivalent between groups. The proportion of high-quality blastocysts (≥3BB; UFW: 84.4±29.0%, TW: 81.3±22.8%; p = 0.791) was also comparable between the two groups. Limitations, reasons for caution This retrospective, single-center study has a moderate sample size. Larger, multicentric studies are needed to validate these findings. The recent introduction of the Ultra-Fast Warming protocol precludes live birth outcome reporting. While the cryotransfer sample size is robust, the number of oocyte thawing cycles remains relatively small at this stage. Wider implications of the findings Ultra-Fast Warming demonstrates comparable results to the traditional protocol and offers a promising alternative for human samples. Its ability to maintain outcomes while streamlining processes suggests potential benefits for improving laboratory efficiency and resource management in assisted reproduction practices. Trial registration number No