Increased blastomere number is associated with higher live birth rate in day 3 embryo transfer

PURPOSE: To study the relationship between blastomere number and pregnancy outcomes of day 3 embryo transfers. METHODS: This retrospective cohort study included 2237 fresh single day 3 embryo transfer cycles from October 2013 to November 2020. Patients were divided into six groups according to the blastomere number on day 3: ≤ 6-cell (n = 100), 7-cell (n = 207), 8-cell (n = 1522), 9-cell (n = 187), 10-cell (n = 91) and ≥ 11-cell (n = 130). Generalized estimating equation analysis based on multivariate logistic regression model was performed to adjust for potential confounders. RESULTS: The live birth rate (LBR) was 19.0%, 27.1%, 38.9%, 32.1%, 44.0% and 53.8% for the ≤ 6-cell, 7-cell, 8-cell, 9-cell, 10-cell and ≥ 11-cell groups, respectively (P < 0.001). Specifically, the ≤ 6-cell group was associated with reduced LBR compared with the 8-cell group (aOR 0.50, 95% CI 0.29–0.86; P = 0.013). Conversely, the odds of live birth were significantly increased in patients transferred with 10-cell embryos (aOR 1.62, 95% CI 1.03–2.53; P = 0.035) and ≥ 11-cell embryos (aOR 2.14, 95% CI 1.47–3.11; P < 0.001) when using the 8-cell embryo group as reference. Similar trends were also observed in the rates of positive hCG test and clinical pregnancy, while no significant differences were detected in miscarriage risk. CONCLUSION: Increased blastomere number was associated with higher LBR in fresh single day 3 embryo transfer cycles. This finding questions the consensus on the reduced developmental potential of fast-cleaving embryos. Further large prospective studies are warranted for confirmation. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12884-022-04521-5.