Improved clinical outcomes with one-step warming of vitrified blastocysts: a retrospective analysis of 3167 transfers

Does a one-step warming protocol for vitrified blastocysts improve post-warming recovery, clinical outcomes and neonatal safety compared with a conventional multi-step warming method?

A retrospective analysis of 3167 vitrified blastocyst warming cycles compared outcomes between a one-step protocol (n = 1750) and a conventional multi-step protocol (n = 1417). Key outcomes included post-warming survival and re-expansion, biochemical and clinical pregnancy rates, ongoing pregnancy rate, live birth rate, neonatal parameters (gestational age, birth weight) and miscarriage rate. Subgroup analyses were performed in Society for Assisted Reproductive Technology age groups and preimplantation genetic testing for aneuploidy (PGT-A) cycles.

Post-warming survival exceeded 99% in both groups. Early re-expansion (0-2 h) was higher for the multi-step warming protocol, but by 4 h, the one-step warming protocol showed better expansion in the total cohort. The one-step warming protocol achieved significantly higher rates of β-human chorionic gonadotrophin positivity (77.7% versus 73.5%), clinical pregnancy (72.1% versus 67.2%), ongoing pregnancy (66.5% versus 61.2%) and live birth (63.7% versus 57.0%; all P < 0.001) in the total cohort. In patients aged <35 years, ongoing pregnancy and live birth rates were significantly higher with the one-step warming protocol (68.9% versus 64.0%, P = 0.04; 66.8% versus 59.6%, P = 0.003). In the same age group with PGT-A, clinical pregnancy improved significantly (78.8% versus 71.9%; P = 0.019). Live births across age groups were numerically higher with the one-step warming protocol, although the differences did not achieve significance, and neonatal outcomes were comparable between protocols.

One-step warming is a safe and effective alternative to conventional multi-step warming. It improves clinical outcomes, particularly in younger patients and euploid embryo transfers, while maintaining neonatal safety and streamlining laboratory workflows.