#307 : Establishment of an Early Blastocyst Stage Classification System and Its Implications for Clinical Pregnancy and Live Birth Outcomes

Background and Aims: Sub-optimal embryo development on day 5 often leads to the transfer of an early blastocyst (EB) stage embryo, especially for women >38 years of age. Yet there are few studies of reported outcomes, nor an industry-wide EB classification system to identify EBs for fresh transfer with the best outcomes. Hence, this study aimed to determine clinical outcomes and whether an EB-stage classification system could be beneficial. Method: A retrospective cohort study analysed 537 cycles undertaken between 2019 and 2022 at two large clinics, in which patients had on day-5 a single fresh EB stage embryo transferred. A novel EB classification system was developed and applied to determine clinical pregnancy rate (CPR) and live birth rate (LBR) outcomes, as well as the effects of maternal age ([Formula: see text] or > 38 years of age). EB stage embryos were classified into four categories based on day-5 morphological features from highest (EB-A) to lowest (EB-D) quality. Data were analysed using Fisher’s exact and non-parametric analyses. Results: Transfer of a fresh EB embryo resulted in a 20.1% CPR and 14.7% LBR. Classification of EBs into four distinct sub-groups identified differences in CPR and LBR (LBR: EB-A 23.4%; EB-B 10.3%; EB-C 12.9%; EB-D 7.6%). Similar CPR and LBR trends were determined for patients [Formula: see text] 38 years of age (LBR:26.7%,14.2%,19.0%, 2.1%) and >38 years of age (LBR:17.5%,4.3%,5.8%,2.7%), with an overall lower LBR for women >38 years of age (8.0%) compared with [Formula: see text] 38 years old (19.5%; P<0.05). Conclusion: Whilst pregnancies and live births can result from an EB transfer, LBR varied by >15% between different morphological classifications. If required, transfer of a late-stage EB results in a higher LBR, especially for women [Formula: see text] 38 years of age. Use of an EB classification system would benefit clinical decisions around when to transfer an EB stage embryo.