Impact of Day 3 Cell Number on the Live Birth Rate of Vitrified-Warmed Single Blastocyst Transfer

Abstract

Abstract Background: Previous studies have reported inconsistent results regarding blastocyst selection with high cell number of day 3 (D3) and the eventual pregnancy outcomes. Thus, this study investigated the relationship between the D3 cell number and clinical outcomes of single blastocyst transfer (SBT) in vitrified-warmed transfer cycles. Methods: A total of 1144 vitrified-warmed cycles of SBT were retrospectively analyzed. Based on the D3 cell number, the cycles were divided into four groups: group A (3–7 cells, n = 130); group B (8–9 cells, n = 621); group C (10–12 cells, n = 328); and group D (13–16 cells, n = 65). The differences in live birth rate (LBR), clinicalpregnancy rate, and miscarriage rate were examined among the four groups. Results: The LBR and clinical pregnancy rate increased with the D3 cell number (P < 0.01). No significant difference was found in the miscarriage rate among the groups (P= 0.055). After adjusting for confounding factors, the LBR was significantly higher in groups C (odds ratio [OR] = 1.506, 95% confidence interval [CI]: 1.148–1.976, P < 0.01) and D (OR = 2.081, 95% CI: 1.218–3.556, P< 0.01) than in group B. Conclusions: A high D3 cell number (>9 cells) was associated with a high LBR in the vitrified-warmed SBT cycle. The cell number of D3 embryos can be an important reference indicator for blastocyst selection. Among blastocysts with the same morphological score, those with >9 cells on D3 can be preferentially selected for transplantation.