Effect of day 3 embryo cell number on the pregnancy and neonatal outcomes of day 4 single embryo transfer from fresh cycles

Background The aim of this retrospective cohort study was to assess the impact of day 3(D3) embryo cell number on the clinical pregnancy and neonatal outcomes of day 4(D4) single embryo transfer in fresh cycles. Methods The study included 431 day 4 single embryo transfer in fresh cycles conducted between December 2018 and June 2023. These cycles were divided into three groups according to the day 3 embryo cell number: 248 cycles in the 7 ~ 9-cell group, 149 cycles in the 10 ~ 13-cell group and 34 cycles in the >13-cell group, and clinical pregnancy outcomes and neonatal outcomes were compared among the three groups. Results The clinical outcomes with 10 ~ 13-cell were significantly higher than those of the 7 ~ 9-cell group, regardless of whether the female age was < 30 or ≥ 30 years. The same result could be found when the insemination pattern was IVF, and when the transferred embryos were the grade of complete fusion. There were no differences in neonatal outcomes between different groups.After adjusting for confounding factors, the 7 ~ 9-cell group was associated with lower clinical pregnancy rates and live birth rates compared with the 10 ~ 13-cell group (clinical pregnancy rates: aOR 0.521, 95% CI -1.198~-0.105, P  = 0.019; LBR: aOR 0.539, 95% CI -1.122~-0.114, P  = 0.016) Conclusion The cell number of D3 embryos can be an important reference indicator for D4 embryo selection. When performing day 4 single embryo transfer in fresh cycles, embryos with 10 ~ 13-cell on D3 can be preferentially selected for transplantation to enhance clinical outcomes, especially when the insemination pattern is IVF,and when the transferred embryos are the grade of compaction stage