Association between pretransfer cleavage-stage blastomere dynamics and pregnancy outcomes in fresh single embryo transfer cycles: a retrospective cohort study

Qingkai Wang¹Shuangshuang Geng¹Hansheng Feng¹Bin Zhang¹Kai Deng¹Zimo Zhao²Jinfeng Xu¹Haiye Wang¹Jing Wang¹Weimin Yang^1*Liyi Cai^1*

• ¹Hebei Reproduvtive Maternity Hospital, Shijiazhuang, China
• ²China Agricultural University College of Biological Sciences, Beijing, China

Objective: To evaluate the impact of embryo blastomere cell number and dynamic changes in cell number during the morning of embryo transfer day (7:00–11:00) on clinical pregnancy outcomes in fresh single embryo transfer (SET) cycles. Methods: A retrospective cohort study was performed, including 561 fresh SET cycles conducted between January 2022 and June 2024. Cycles were categorized into four groups based on embryo blastomere count before transfer: ≤7-cell, 8-cell, 9–10-cell, and ≥11-cell groups. We analyzed the relationship between the increase in blastomere number observed from 7:00 a.m. to 11:00 a.m. on the day of transfer and clinical pregnancy outcomes. Multivariate logistic regression analysis was utilized to assess the influence of various factors on clinical pregnancy and live birth rates. Results: Clinical pregnancy rates significantly differed among the ≤7-cell, 8-cell, 9–10-cell, and ≥11-cell groups (10.64%, 36.69%, 42.31%, and 46.32%, respectively; P=0.004). Live birth and biochemical pregnancy rates exhibited a similar increasing trend with higher cell numbers (P=0.001), whereas early miscarriage rates showed no significant differences among groups (P=0.157). In the 9–10-cell group, embryos that exhibited an increase in blastomere number had significantly higher clinical pregnancy rates (50% vs. 23.68%, P=0.006) and live birth rates (41.30% vs. 15.79%, P=0.005). No significant differences were observed in the ≤7-cell and 8-cell groups (P > 0.05). Multivariate logistic regression analysis demonstrated that increased endometrial thickness significantly improved clinical pregnancy likelihood (P=0.034), whereas lower blastomere number (≤7-cell) significantly reduced clinical pregnancy rates (P=0.002). Conclusion: A higher embryo blastomere count before transfer is significantly associated with improved clinical pregnancy and live birth outcomes in fresh SET cycles. Short-term increases in blastomere number on the morning of transfer day may reflect superior embryo developmental potential.