P-123 Clinical outcomes following transfer of blastocysts that had fully developed by Day 4

Abstract Study question Are blastocysts that have fully developed by Day 4 viable based on clinical outcomes following subsequent transfer? Summary answer The clinical outcomes following the transfer of fully developed Day 4 blastocysts demonstrate similar clinical outcomes to Day 5 fully developed blastocysts. What is known already In human assisted reproductive technology (ART), embryos that reach the full blastocyst stage by Day 4 (FBD4) have shown high pregnancy rates (Ozgur et al., 2021). Additionally, we have demonstrated that embryos developed to FB day 4 exhibit a high rate of euploidy (Kato et al., ESHRE 2024). However, the clinical outcomes following their transfer remains unexplored. This study aimed to assess the clinical utility of vitrified, euploid FBD4, subsequently transferred on Day 5, based on pregnancy, miscarriage rates, and birth outcomes. Study design, size, duration This retrospective study included 145 cycles from 120 patients of euploid FBD4 embryos, all of whom subsequently underwent embryo transfer on Day 5 of a frozen embryo transfer cycle. They had undergone trophectoderm biopsy and preimplantation genetic testing for aneuploidy (PGT-A) between January and December 2022. Participants/materials, setting, methods Embryos were divided into two groups: those that developed to blastocyst (Gardner’s: ≥ expansion grade 3) on Day 4 (96-100 hours) (FBD4(+) group) and those that did not reach the full blastocyst stage by Day 4 (FBD4(-) group), but developed to blastocyst by Day 5. Clinical and birth outcomes including congenital abnormalities were compared between groups. Statistical analysis used Fisher’s exact test and Mann-Whitney U test to assess significant differences (P < 0.05). Main results and the role of chance The clinical pregnancy rate in the FBD4(+) group was 52.5% (31/59), which was not significantly different from the 47.7% (41/86) in the FBD4(-) group. The miscarriage rates were also similar (22.6% vs. 24.4%). There were no significant differences in pregnancy rates (50.0% vs. 47.7%) or miscarriage rates (10.0% vs. 29.0%) between women aged 39 or younger and those aged 40 or older (57.9% vs. 47.6%, 45.5% vs. 10.0%, respectively). The male-to-female ratio was 14:10 in the FBD4(+) group and 16:9 in the FBD4(-) group. Birth weights were 3028.9 ± 747.2 g for boys and 2787.4 ± 932.8 g for girls in the D4(+) group, and 3052.9 ± 532.3 g for boys and 3068.4 ± 263.5 g for girls in the D4(-) group, with no significant differences. The number of days to conception was 264.8 ± 27.1 days in the D4(+) group and 274.6 ± 13.6 days in the D4(-) group, with no significant difference. Apgar scores were 8.0 ± 1.0 for the D4(+) group and 7.8 ± 1.7 for the D4(-) group, with no significant difference. The rate of congenital anomalies was 4.2% (1/24) in the D4(+) group and 4.0% (1/25) in the D4(-) group, with no significant difference. Limitations, reasons for caution This was a retrospective study limited to day 5 embryos with biopsies taken after vitrification-warming. Wider implications of the findings Embryos that reached the FBD4 showed comparable pregnancy rates, miscarriage rates, and birth outcomes to embryos that had not. Therefore, earlier developing blastocysts identified as euploid can be transferred without compromising outcomes. This approach could potentially reduce PGT-A costs and lead to a wider window for choosing embryos for transfer. Trial registration number No