Live births following universal single-step warming across multiple vitrification systems

To report live birth and neonatal outcomes across multiple commercial kit brands, including cross-brand combinations, thereby providing clinical validation of a universal warming method on the basis of single-step elution using 1 M nonpermeating solutes (NPS). Human blastocysts are routinely vitrified using commercial kits with minor compositional differences. Previous studies have shown that simplified single-step warming with 1 M NPS can reduce handling time and improve laboratory efficiency. However, this universal ultrafast warming approach has not been systematically evaluated across different warming kits or mixed vitrification-warming combinations.

Retrospective observational study.

A total of 469 vitrified blastocysts warmed between January 1, 2024 and December 31, 2024.

Blastocysts were vitrified using four commercially available kits and warmed by single-step elution in 1 M NPS from the corresponding thawing solutions, generating 12 vitrification-warming combinations. Vitrification was performed at room temperature (12-15 min in equilibration solution followed by 60 s in vitrification solution before plunging). Warming consisted of a single step in thawing solution (1 M NPS) at 37 °C.

Cryosurvival, clinical pregnancy, implantation, live birth and neonatal outcomes.

Overall, 469 blastocysts were warmed and transferred in 428 cycles, resulting in 100% survival, a clinical pregnancy rate of 45.8%, an implantation rate of 43.1%, and a live birth rate of 36.0%, with 159 babies born. No significant differences were observed between groups in pregnancy, implantation or live birth rates. Mean neonatal weight was 3.2 ± 0.4 kg and was comparable between groups, ranging from 3.0 ± 0.7 to 3.6 ± 0.5 kg, and similar to that observed following fresh embryo transfers during the same period. The preterm live birth rate was 6.3%, with no difference between groups, and was comparable with fresh transfers and national Assisted Reproductive Technology registry data. Sex distribution was balanced (48.4% male, 51.6% female) and comparable between groups, fresh transfers (46.9% male, 53.1% female), and national registry data. No major congenital anomalies or severe neonatal diseases were reported. Some between-group comparisons are limited by small sample sizes and high survival rates, so nonsignificant P values may be due to low statistical power rather than true group similarity.

This case series of babies born following universal single-step warming (USSW) provides preliminary reassuring evidence supporting the feasibility of this protocol across different commercial vitrification and warming systems, despite compositional differences. However, given the limited sample size of some subgroup combinations, further adequately powered studies are required to confirm its safety and clinical effectiveness. If validated, this approach may help streamline laboratory practice, reduce costs, and broaden access to in vitro fertilization.