Embryonic Outcomes, Live-Birth Outcomes After Embryo Transfer, and Euploid Rates for Various Direct Cleavage Timing Definitions During First Cytokinesis: A Single-Center, Large-Cohort, Retrospective Study.

Although essential to confirm the clinical utility of direct cleavage embryos, the timing of rapid cleavage (RpiC) has not been specifically defined. This study aimed to explore the differences in embryonic and clinical outcomes based on varying timing parameters of direct cleavage during first cytokinesis, using time to reach the three-cell stage (t3) minus time to reach the two-cell stage (t2). We analyzed 19,796 fertilized embryos (in 6,907 patients) from a single center between September 2019 and December 2020. The embryos were cultured using EmbryoScope, and t2 and t3 were recorded. Trichotomous mitosis (TM) was defined as t3 - t2 = 0 h, and RpiC events were divided into four groups, as follows: 0 h < t3 - t2 < 1 h (RpiC-1), 1 h ≤ t3 - t2 < 3 h (RpiC-3), 3 h ≤ t3 - t2 < 5 h (RpiC-5), and 5 h ≤ t3 - t2 < 7 h (RpiC-7). Additionally, 7 h ≤ t3 - t2 < 14 h was defined as normal cleavage. After single-cleavage embryo transfer, the live-birth, TM, and RpiC-1 rates were significantly lower than those in other groups. Similarly, when blastocysts were utilized, the TM and RpiC-1 rates were significantly lower than those in other groups. This study suggests that embryos with TM or RpiC-1 (t3 - t2 < 1 h) should be cultured to the blastocyst stage to prevent unnecessary embryo transfers, although outcomes may vary in different scenarios, i.e., by institution and patient.