Improved embryology outcome and morphokinetic characteristics of embryos derived from frozen/thawed oocytes injected 3 versus 2 hours post-warming

Abstract

To assess whether the duration of recovery following the warming of cryopreserved oocytes affects the morphological and morphokinetic properties of the developing embryo. This is a retrospective data analysis examining the effect of the post-warming recovery duration of vitrified oocytes prior to ICSI. Frozen oocytes from 36 patients (July 2018 to March 2019), either autologous or donor, were included in the study. The study examined cycles in which the oocytes were injected within a 2-hour recovery period and compared them to those in which the oocytes were injected within 3 hours or more within the same patient oocyte cohort. All injected oocytes were cultured individually in time-lapse incubators (TLM; EmbryoScope, Vitrolife, Sweden). Embryo transfers occurred on D3 or D5 regardless of the study groups. Standard oocyte freezing protocol included vitrification 2 hours after retrieval using a Kitazato-based (Japan) vitrification media. Oocyte warming was performed using Kitazato thaw media. ICSI was performed in the standard fashion, and embryos were annotated daily for their developmental hallmarks. Embryo selection for ET or freezing was performed by standard laboratory protocols. No significant differences were observed in 2PN and/or abnormal fertilization rates between the groups. Similarly, on day 3, no significant differences were found in the average cell numbers or in the average number of good (≥8 cells, <20% frag.), fair (5-7 cells, any abnormal cleavages), and poor (≤4 cells) embryos. Equally, the incidence of abnormal cleavage patterns was similar among the cohort. More embryos were selected for transfer from the 3-hour group than from the 2-hour group (51.1% vs. 48.9%; p=0.78). There were no significant differences in the utilization rate (the number of embryos transferred fresh plus blastocysts frozen) between the 3- and 2-hour groups (49.6% vs. 49.6%, respectively). The time-lapse morphokinetic data indicated faster-growing embryos in the 3- versus 2-hour group. This difference is apparent in late-stage morphokinetic parameters (t9 to tsB). The 3-hour group produced significantly better quality blastocysts in cycles that were either frozen upfront, fresh D5 ET, or PGT (p=0.03). The duration of the recovery time post-warming showed no significant differences in overall embryology outcomes prior to blastocyst formation. However, at the BL stage, the 3-hour group demonstrated improvement in morphokinetic parameters and in overall BL quality.