First cleavage signatures define a temporal trade-off and predict embryo fitness

Early embryonic development is marked by substantial variability in cleavage dynamics, yet the extent to which first cleavage (FC) features relate to subsequent developmental progression remains incompletely defined. Using parthenogenetic (haploid and diploid) and fertilized embryos from Swiss albino mice, we combined time-lapse imaging with mechanistic analyses to define a novel FC-state grading system (FC-I to FC-IV) based on cleavage symmetry, fragmentation, and correction of the cleavage axis. Aberrant FCs arose from cytoskeletal instability, persistent cytoplasmic bridges, chromosome lagging, and transient nuclear mispositioning. We identified a reproducible inverse relationship between FC duration and subsequent cleavage interval, that predicts blastocyst quality and termed it as 'first cleavage-associated compensatory timing (FACT) relationship'. Haploid parthenogenetic embryos lacked this compensatory timing relationship, which was observed in fertilized embryos and diploid parthenotes, coinciding with markedly reduced developmental progression and competence. Analysis of > 1,100 human embryos revealed analogous FC-states and an inverse relationship between consecutive cleavage intervals that was associated with blastocyst morphology and developmental progression, with no clear association with aneuploidy. These findings position FC-state grading and the FACT relationship as potential non-invasive indicators of early embryo fitness.