Abstract
OBJECTIVE: The aim of this retrospective cohort study was to evaluate the clinical and neonatal outcomes between day 4 and day 5 embryo transfer across different fertilization methods. METHODS: This retrospective cohort study was conducted between January 2018 and March 2023, enrolling a total of 1,245 fresh embryo transfer cycles, including 1,023 in vitro fertilization (IVF) cycles and 222 intracytoplasmic sperm injection (ICSI) cycles. Among these, IVF cycles included 793 day 4 transfers and 230 day 5 transfers; ICSI cycles included 179 day 4 transfers and 43 day 5 transfers. The study conducted a comparative analysis of clinical pregnancy outcomes between day 4 and day 5 transfers. RESULTS: In IVF or ICSI cycles, no significant differences were found in clinical pregnancy rate (CPR), implantation rate (IR), live birth rate (LBR), or other clinical outcomes between transfers of day 4 and day 5 embryos (P > 0.05). In IVF cycles with single high-quality embryo transfer, the CPR (62.72%, P = 0.026), gestational week of delivery (39 weeks, P = 0.026) of day 4 were significantly higher than day 5 (38 weeks). After controlling for potential confounding factors, the CPR of the day 4 group was also higher than day 5 (OR 0.578, 95% CI 0.352-0.949, P = 0.030). In IVF or ICSI cycles with day 4 high-quality embryo transfer, the LBR (63.06%, P = 0.006; 70.37%, P = 0.006) and multiple pregnancy rate (MPR; 53.29%, P < 0.001; 50%, P < 0.001) of transferring double high-quality embryo were significantly higher than transferring single high-quality embryo (LBR: 50.87%, 44.23%; MPR: 0%, 3.23%). CONCLUSION: In IVF or ICSI cycles, day 4 embryo transfer is considered a viable option or alternative to day 5 blastocyst transfer with no difference in clinical and neonatal outcomes. In IVF cycles with single high embryo transfer, day 4 transfer is recommended due to its significantly higher pregnancy rate compared to day 5 transfer. To reducing MPRs and preterm birth rates (PBRs), day 4 single embryo transfer is recommended if embryos achieve high-quality grade (full compaction) on day 4.