The degree of blastocyst re-expansion immediately after warming is a strong dynamic indicator of embryo quality

Does the degree of blastocyst re-expansion at 9-11 min after warming affect vitrified-warmed blastocyst transfer cycle outcomes, and could it be a quantitative measure suitable for clinical application in assessing embryo quality?

This retrospective observational cohort study included 1356 cases of vitrified-warmed blastocyst transfer cycles at the Reproduction Center, Toho University Omori Medical Center between January 2017 and December 2022. The degree of re-expansion was calculated using image-analysis software. The degree of blastocyst re-expansion at about 9-11 min after warming and its association with cycles outcomes were evaluated. Clinical pregnancy was the primary outcome.

Blastocysts were grouped at 9-11 min after warming based on a calculated cut-off value for the degree of re-expansion (90.2%): the 're-expansion group' (≥90.2% without hatching), the 'shrinkage group' (<90.2%) and the 'hatching group'. Implantation, clinical pregnancy, ongoing pregnancy and live birth rates were significantly higher in the re-expansion versus shrinkage group (P < 0.001) and were comparable with the hatching group. Logistic regression analysis revealed age, the degree of blastocyst re-expansion 90.2% or higher, inner cell mass quality and trophectoderm quality as independent predictors of clinical pregnancy, whereas additional variables showed no significant adjusted odds ratios.

The degree of blastocyst re-expansion at 9-11 min after warming is a strong dynamic indicator of embryo quality, offering a novel metric distinct from traditional morphological assessments. This study highlights the importance of rapid re-expansion as a predictor of clinical pregnancy and underscores its potential utility in enhancing the selection process for embryo transfer.