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Background/Objectives: Early embryonic arrest during the cleavage stage (days 2–4) accounts for a substantial proportion of developmental failure in in vitro fertilization. This phenomenon remains poorly understood at the molecular level, even in chromosomally normal embryos identified by preimplantation genetic testing. This review aims to redefine cleavage-stage arrest from a passive energy deficit to a checkpoint-regulated endpoint caused by inadequate coordination among metabolism, transcriptome integrity, and stress-response pathways. Methods: We integrate evidence from long-read transcriptomics, metabolomics, epigenetics, and immunobiology relevant to pre-blastocyst development. These data are assembled into a unifying mechanistic framework and a clinically oriented stratification model, together with candidate multimodal readouts for early classification. Results: We propose a three-axis model linking: (i) metabolic–epigenetic insufficiency, including defective histone lactylation and impaired alpha-ketoglutarate-dependent DNA demethylation; (ii) isoform-level abnormalities, including intron retention and retrotransposon activation within a hidden transcriptomic landscape better resolved by long-read sequencing; and (iii) stress-related immune signaling, in which NLRP7 links alternative splicing and DNA-damage-response dysfunction with mitochondrial stress and p53-associated arrest. Within this framework, we distinguish three molecular arrest states: an early transition failure marked by defective maternal-to-embryonic reprogramming and severe splicing disruption; a metabolically quiescent state that may retain a limited rescue window; and a later stress-associated state characterized by senescence-like features, oxidative stress, and broad transcriptomic and genomic instability. Conclusions: Early embryo arrest should no longer be viewed as a nonspecific developmental failure, but as a mechanistically stratifiable condition with distinct metabolic, transcriptomic, and stress-associated trajectories. A diagnostic platform combining fluorescence lifetime imaging microscopy, long-read sequencing, and digital polymerase chain reaction may improve early mechanistic classification, help identify embryos with possible reversibility, and reduce uncertainty in embryo selection during in vitro fertilization.