Temporal dynamics of Spontaneous Blastocyst Re-expansion as A Predictor of Frozen Embryo Transfer Success

Abstract Post-thaw blastocyst collapse is a frequent occurrence in frozen embryo transfer (FET) cycles, yet standard evaluation often relies on static post-thaw snapshots. This retrospective cohort study evaluated 438 single-blastocyst FET cycles using good-quality day 5 blastocysts vitrified without artificial shrinkage, introducing a dynamic evaluation framework based on two routine clinical timepoints: immediately post-thaw (t1) and pre-transfer (t2). Our results demonstrate that expansion status at transfer, rather than initial post-thaw appearance, is the primary predictor of success. Blastocysts that re-expanded prior to transfer achieved live birth rates (22.7%) comparable to continuously expanded embryos (20.0%; p>0.05), whereas persistent collapse resulted in significantly lower success (7.8%; p<0.03). Crucially, a significant interaction was revealed between initial collapse and the thaw-to-transfer interval: for every additional hour of incubation, the odds of remaining collapsed significantly decreased (OR=0.49 ,p=0.038). These findings demonstrate that spontaneous collapse is a transient, reversible state that can be successfully overcome through extended post-thaw incubation. Assessing embryos across two simple, routine timepoints provides clinicians with an accessible framework to manage incubation timing, maximize re-expansion probability, and optimize live birth outcomes. Lay Summary When embryos are frozen and thawed for fertility treatment, they sometimes collapse, but does this mean they are no longer viable? This study followed 438 frozen embryo transfer cycles to understand whether collapsed embryos can recover, and whether giving them more time in the incubator before transfer makes a difference. The key finding is that what matters most is how an embryo looks at the time of transfer, not immediately after thawing. Embryos that re-expanded before being transferred had similar success rates to those that never collapsed. Only embryos that remained collapsed at transfer had significantly poorer outcomes. Importantly, for every additional hour spent in the incubator, a collapsed embryo's odds of recovering improved substantially. This means that rather than discarding a collapsed embryo, clinicians may simply need to wait, giving it time to recover could meaningfully improve the chances of a successful pregnancy. Collapsed embryos that are given longer to recover in the incubator before transfer may have the same chance of achieving a pregnancy as embryos that never collapsed.