Does the day 3 embryo cell number affect pregnancy and neonatal outcomes following single blastocyst transfer from frozen embryo transfer cycles?

Abstract

Abstract Background:The aim of this retrospective cohort study was to assess the impact of day 3 embryo cell number on the clinical pregnancy and neonatal outcomes of single blastocyst transfer in frozen embryo transfer (FET) cycles. Methods:The study included 624 day 5 single blastocyst FET cycles conducted between January 2017 and December 2022. Patients were categorized into four groups based on the cell number on day 3: 51 cycles in the <7-cell group, 378 cycles in the 7~9-cell group, 121 cycles in the 10~13-cell group, and 74 cycles in the >13-cell group. The study compared the clinical pregnancy outcomes and neonatal outcomes among the four groups. Results: The clinical pregnancy rates of blastocysts in the 7~9-cell group and the 10~13-cell group were significantly higher than those in the <7-cell group. Additionally, the live birth rates in the 7~9-cell group were significantly higher than those in the <7-cell group. While the clinical pregnancy and live birth rates in the >13-cell group decreased compared to the 7~9-cell group and the 10~13-cell group, the differences were not significant. There were no significant differences in the clinical pregnancy rate and live birth rate between the 10~13-cell group and the 7~9-cell group. Additionally, there were no significant differences in the rates of ectopic pregnancy cycles, miscarriage, monozygotic twins, premature delivery, gestational week of delivery, male/female ratio, and birth weight among the four groups.After adjusting for potential confounding factors, the <7-cell group was associated with lower clinical pregnancy and live birth rates compared to the 7~9-cell group and the 10~13-cell group. The clinical pregnancy and live birth rates of the >13-cell group were lower than those of the 7~9-cell group. Subgroup analysis revealed significant differences in the clinical pregnancy and live birth rates among the overall groups for individuals aged ≤30, and the insemination pattern of ICSI. Conclusion: When performing single blastocyst transfer in FET cycles, blastocysts with 7~13 cells on day 3 should be prioritized, to enhance clinical pregnancy outcome, especially when patients are aged ≤30 years, and when the insemination pattern is ICSI.