Simplified warming of vitrified human embryos: comprehensive analysis of diverse morphological qualities and...

Is a simplified one- or two-step warming protocol effective and safe for vitrified human embryos across various developmental stages, morphological qualities, and days of blastulation?

This retrospective cohort study, conducted at a single IVF centre, analysed 1501 single-embryo transfer cycles (150 cleavage-stage embryos, 1351 blastocysts) performed between January and December 2024. Outcomes were compared between a conventional three-step protocol (thawing solution for 1 min, dilution solution for 2 min and washing solution for 3 min; n = 742) and simplified protocols: two-step (thawing solution for 1 min and dilution solution for 2 min; n = 461) and one-step (thawing solution for 1 min; n = 298). Separate analyses were conducted for cleavage-stage embryos and blastocysts. For blastocysts, a multivariable logistic regression and subgroup analyses were further performed based on developmental stage, morphological quality, and day of blastulation.

For cleavage-stage embryos, all protocols resulted in 100% survival and comparable ongoing pregnancy rates (11.1% for three-step protocol, 12.5% for two-step protocol, 10.8% for one-step protocol; P = 1.0000). For blastocysts, the post-warming survival rate (100%, 99.3%, 99.6%; P = 0.1112) and ongoing pregnancy rate (47.7%, 46.2%, 43.9%; P = 0.5765) were comparable among the three protocols, including when confounders were considered. This equivalence was observed consistently across all subgroups, including developmental stage (full blastocyst: 43.2%, 39.6%, 33.7%, P = 0.3111; expanded blastocyst: 49.6%, 49.5%, 49.1%; P = 0.9950), morphological quality (poor grade: 27.5%, 23.5%, 23.4%, P = 0.7636; good grade: 52.1%, 51.6%, 48.4%, P = 0.6390), and day of blastulation (day 5: 52.7%, 52.9%, 50.3%, P = 0.8234; day 6: 34.4%, 28.5%, 27.4%, P = 0.4088).

Simplified one- or two-step warming protocols are effective for vitrified human embryos, regardless of developmental stage, morphological quality, or day of blastulation.