Abstract
BACKGROUND: The aim of this retrospective cohort study was to evaluate the effect 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 1220 single blastocyst transfer from FET cycles conducted between January 2017 and April 2024. Patients were categorized into four groups based on day 3 embryo cell number : 110 cycles in the < 7-cell group, 743 cycles in the 7-9-cell group, 282 cycles in the 10-13-cell group, and 84 cycles in the > 13-cell group. The study compared the clinical pregnancy outcomes and neonatal outcomes among the four groups. RESULTS: When the maternal age was < 35 years or high-quality blastocysts were transferred, the clinical pregnancy rate (CPR) of the < 7-cell group was significantly lower than those of other three groups (all P < 0.008). Similarly, when high-quality blastocysts were transferred, the live birth rates (LBRs) of the 7-9-cell group and > 13-cell group were significantly higher than those of the < 7-cell group (all P < 0.008). In women aged < 35 years with high-quality blastocyst transfers, after adjusting for confounders, 7-9 and 10-13 groups were with significantly higher CPR (aOR 2.66, 95% CI 1.44-4.91; aOR 2.15, 95%CI 1.10-4.18 and LBR (aOR 2.50, 95%CI 1.32-4.73, aOR 1.74, 95% CI 0.88-3.46). CONCLUSION: In FET cycles, a low day 3 cell number (< 7-cell) on day 3 was related to decreased CPR and LBR after blastocyst transfer, whereas a number > 9 was comparable to that of 7-9 cells.