P-196 Early blastocyst formation on day 4 is associated with improved clinical outcome and advocates for embryo quailty assessment on day 4

Abstract Study question Is early cavitation on day 4 associated with improved clinical outcome? Summary answer Early blastulation on day 4 is a positive predictor of implantation after single blastocyst transfer on day 5. What is known already In order to select the blastocyst with the highest implantation potential for transfer or freezing, several morphological and/or morphokinetic parameters have been proposed. For instance, early compaction on day 3 (D3) or blastocyst expansion on day 5 (D5) have been reported to be relevant when selecting embryos for transfer. However, day 4 has often been overlooked and very few studies focused on early blastulation on day 4 (D4). Indeed, data on early blastulation on day 4 is scarce. It was however recently demonstrated that embryos transferred on day 4 could provide acceptable pregnancy rates. Study design, size, duration This monocentric retrospective cohort study was conducted in all consecutive couples undergoing IVF cycles in 2017-2022 with single blastocyst transfer and observation on day 4. Participants/materials, setting, methods Morphological and morphokinetic assessment was performed by 2 operators at 93±2h, 116±2h (and 140±2h) post insemination, on day 4, 5 (and 6) respectively. A total of 26 507 D4 embryos were included in this study and grouped according to the observation of early cavitation or not : 23 244 were in the D4 control group (absence of early cavitation on D4) and 3263 were in the D4 blastocyst group (observation of early cavitation on D4). Main results and the role of chance The proportion of embryos used clinically for either transfer or freezing on day 5 or 6 was significantly higher in the D4 blastocyst group than in D4 control group (86.7% vs 50.1% respectively, p < 0.05). Concerning pregnancy rate, cinical pregnancy rate was significantly higher in the D4 blastocyst group than in D4 control group (p < 0.05). In the D4 control group, 5051 embryos were transferred and clinical pregnancy rate is 28.7%. In the D4 blastocyst group, 1286 embryos were transferred (fresh or frozen) and clinical pregnancy rate is 40.2%. Limitations, reasons for caution The retrospective and monocentric design calls for caution when considering the generalizability of our findings. Wider implications of the findings We postulate that D4 embryo quality assessment should be widely implemented in any blastocyst program in order to improve embryo quality assessment and subsequently clinical outcome. Trial registration number not applicable