Cleavage pattern, morula compaction and blastocyst morphology as determinants of live birth after single blastocyst transfer

Introduction: This retrospective time lapse study evaluated 3,103 transferred autologous blastocysts to determine how early division patterns, morula compaction behavior and blastocyst quality influence clinical outcomes. Methods: Embryos were categorised by cleavage pattern (normal or abnormal), degree of morula compaction (full or partial), and blastocyst quality (top, good or low). Result: Most transferred blastocysts, 92.5%, originated from normally dividing embryos, of which 63.8% developed into fully compacted morulas. In unadjusted analyses, fully compacted morulas resulted in higher pregnancy, clinical pregnancy and live birth rates than partially compacted morulas across all morphology categories. Embryos with abnormal cleavage constituted 7.5% of the cohort, developed almost exclusively into partial morulas, and showed reduced reproductive potential, with lower pregnancy and clinical pregnancy rates compared with normally dividing embryos, and lower live birth rates compared with partial morulas originating from normal cleavage. The highest live birth rate (38.9%) was observed for top quality blastocysts originating from normally cleaving, fully compacted morulas. In multivariable models adjusting for maternal age and blastocyst developmental day, blastocyst morphology and blastocyst age were the strongest independent predictors of clinical outcome, while maternal age showed a consistent negative association. Abnormal cleavage remained associated with reduced pregnancy and clinical pregnancy rates, although this effect did not persist for live birth, and compaction pattern did not retain significance after adjustment. Discussion: Overall, early developmental behavior, particularly cleavage pattern and morula compaction, aligns with downstream morphology to shape embryo competence, while blastocyst morphology and blastocyst developmental day remain the primary determinants of live birth after single blastocyst transfer.