Authors: Khalied Kaskar, Kristin Sieren, Vonda Brabner, Brogan Huneke, Lindsey VerMilyea, Tex VerMilyea, Kaylen Silverberg
OBJECTIVE: Even though embryo vitrification has been performed for nearly two decades, there is no consensus on exposure timing of embryos to vitrification and warming solutions. With embryo exposure timing in equilibration solution (ES) varying from 6-15 minutes, vitrification is time consuming in a busy IVF lab. Warming is a multistep process that can vary from 10-13 min. The aim of this study was to compare reducing the vitrification and warming time and evaluate the blastocyst survival rate and subsequent pregnancy rate.
MATERIALS AND METHODS: This was a retrospective analysis of 754 frozen embryo transfer cycles between January 2022 and March 2023. Embryos were divided into 3 groups: (1) traditional vitrification and warming, (2) traditional vitrification with rapid warming, and (3) reduced ES exposure with rapid warming. Traditional vitrification comprised of 6-10 min exposure to ES followed by 30-60 sec in vitrification solution (VS) (Vit Kit–Freeze NX, Irvine Scientific). Reduced ES comprised of 2min ES followed by 30-60sec VS exposure. Traditional warming comprised of 1min in thawing solution (TS) at 37°C, followed by 4min in dilution solution (DS) and 4min each in 2 drops of washing solution (WS) (Vit Kit-Warm NX, Irvine Scientific). Rapid warming only included 1min in TS before placing the embryo in culture. Main outcome measures were blastocyst survival rate and positive β-HCG pregnancy rate.
RESULTS: The data obtained are depicted in Table 1. No statistically significant differences were noted in the blastocyst survival (99.4%, 98.1%, and 100.0%) and pregnancy rates (65.6%, 71.7%, and 74.6%) between the three protocols. However, the reduced ES with rapid warming showed the highest survival and pregnancy rates.
Table 1: Blastocyst survival and pregnancy between 3 different vitrification and warming protocols
| FET cycles | Embryos thawed | Embryos survived (%) | Positive β-HCG (%) | |
| 6min ES + 13min warm | 492 | 505 | 502 (99.4%)a,b | 322/491 (65.6%)d,e |
| 6min ES + 1min warm | 148 | 160 | 157 (98.1%)a,c | 104/145 (71.7%)e,f |
| 2min ES + 1min warm | 114 | 119 | 119 (100.0%)b,c | 85/114 (74.6%)d,f |
ap=0.13; bp=0.39; cp=0.13; dp=0.17; ep=0.07; fp=0.62
CONCLUSION: Reduced exposure time in ES did not have a negative effect on blastocyst survival and pregnancy rates. Eliminating the need for DS and WS in the warming process is more efficient and yielded good survival and pregnancy. Combining these two innovations to the vitrification and warming process can improve the efficiency in the IVF lab without loss of embryo health or implantation potential.
IMPACT STATEMENT: Reducing the vitrification and warming times can save time while still maintaining good blastocyst survival and pregnancy outcome.