STUDY QUESTION: Does cell number on Day 3 have an impact on pregnancy outcomes in vitrified-thawed single blastocyst transfer cycles? SUMMARY ANSWER: A low Day 3 cell number (≤5 cells) was independently associated with decreased live birth rate (LBR) during single blastocyst transfer cycles in young women. WHAT IS KNOWN ALREADY: Day 3 cell number is an effective predictor of IVF success rates when transferring cleavage stage embryos. However, the association between Day 3 blastomere number and pregnancy outcomes after blastocyst transfer is still unknown. STUDY DESIGN, SIZE, DURATION: A retrospective cohort study of 3543 patients who underwent frozen-thawed single blastocyst transfers from January 2013 to June 2018 at a tertiary-care academic medical center. PARTICIPANTS/MATERIALS, SETTING, METHODS: Patients were grouped into six groups according to the Day 3 cell number: ≤4 cells, 5 cells, 6 cells, 7 cells, 8 cells and >8 cells. The primary outcome measure was LBR. A logistic regression analysis was performed to explore the independent association between Day 3 blastomere number and LBR after adjustment for some potential confounders. MAIN RESULTS AND THE ROLE OF CHANCE: In women <35 years old, the LBR varied significantly according to Day 3 cell number, with the rate of 31.2%, 34.4%, 41.9%, 45.1%, 48.1% and 48.2% for the ≤4-cell, 5-cell, 6-cell, 7-cell, 8-cell and >8-cell groups, respectively (P < 0.001). This significant difference was also observed in the high- and low-quality blastocyst subgroups of young women. However, for women ≥35 years old, the rate of live birth was similar between groups. Furthermore, after accounting for confounding factors, the LBR was significantly decreased in the ≤4-cell (adjusted odds ratio (aOR): 0.62, 95% CI: 0.48-0.80, P < 0.001) and 5-cell (aOR: 0.73, 95% CI: 0.57-0.92, P = 0.009) groups as compared to the 8-cell group. Likewise, the blastocysts arising from ≤4-cell (aOR: 0.73, 95% CI: 0.57-0.93, P = 0.010) or 5-cell (aOR: 0.77, 95% CI: 0.61-0.97, P = 0.024) embryos were associated with lower clinical pregnancy rate than those from 8-cell embryos. No significant differences were observed in biochemical pregnancy rate and miscarriage rate. LIMITATIONS, REASONS FOR CAUTION: A limitation of the current study was its retrospective design. Future prospective studies are needed to confirm our findings. WIDER IMPLICATIONS OF THE FINDINGS: Our observations suggested that a low Day 3 cell number was related to decreased LBR after blastocyst transfer in young women, which provided vital information for clinicians in selecting blastocyst during IVF treatment. STUDY FUNDING/COMPETING INTEREST(S): This study was supported by the National Natural Science Foundation of China (NSFC) (31770989 to Y.W.; 81671520 to Q.C.) and the Shanghai Ninth People's Hospital Foundation of China (JYLJ030 to Y.W.). The authors have no conflicts of interest to declare. TRIAL REGISTRATION NUMBER: N/A.
STUDY QUESTION: Does cell number on Day 3 have an impact on pregnancy outcomes in vitrified-thawed single blastocyst transfer cycles? SUMMARY ANSWER: A low Day 3 cell number (≤5 cells) was independently associated with decreased live birth rate (LBR) during single blastocyst transfer cycles in young women. WHAT IS KNOWN ALREADY: Day 3 cell number is an effective predictor of IVF success rates when transferring cleavage stage embryos. However, the association between Day 3 blastomere number and pregnancy outcomes after blastocyst transfer is still unknown. STUDY DESIGN, SIZE, DURATION: A retrospective cohort study of 3543 patients who underwent frozen-thawed single blastocyst transfers from January 2013 to June 2018 at a tertiary-care academic medical center. PARTICIPANTS/MATERIALS, SETTING, METHODS:Patients were grouped into six groups according to the Day 3 cell number: ≤4 cells, 5 cells, 6 cells, 7 cells, 8 cells and >8 cells. The primary outcome measure was LBR. A logistic regression analysis was performed to explore the independent association between Day 3 blastomere number and LBR after adjustment for some potential confounders. MAIN RESULTS AND THE ROLE OF CHANCE: In women <35 years old, the LBR varied significantly according to Day 3 cell number, with the rate of 31.2%, 34.4%, 41.9%, 45.1%, 48.1% and 48.2% for the ≤4-cell, 5-cell, 6-cell, 7-cell, 8-cell and >8-cell groups, respectively (P < 0.001). This significant difference was also observed in the high- and low-quality blastocyst subgroups of young women. However, for women ≥35 years old, the rate of live birth was similar between groups. Furthermore, after accounting for confounding factors, the LBR was significantly decreased in the ≤4-cell (adjusted odds ratio (aOR): 0.62, 95% CI: 0.48-0.80, P < 0.001) and 5-cell (aOR: 0.73, 95% CI: 0.57-0.92, P = 0.009) groups as compared to the 8-cell group. Likewise, the blastocysts arising from ≤4-cell (aOR: 0.73, 95% CI: 0.57-0.93, P = 0.010) or 5-cell (aOR: 0.77, 95% CI: 0.61-0.97, P = 0.024) embryos were associated with lower clinical pregnancy rate than those from 8-cell embryos. No significant differences were observed in biochemical pregnancy rate and miscarriage rate. LIMITATIONS, REASONS FOR CAUTION: A limitation of the current study was its retrospective design. Future prospective studies are needed to confirm our findings. WIDER IMPLICATIONS OF THE FINDINGS: Our observations suggested that a low Day 3 cell number was related to decreased LBR after blastocyst transfer in young women, which provided vital information for clinicians in selecting blastocyst during IVF treatment. STUDY FUNDING/COMPETING INTEREST(S): This study was supported by the National Natural Science Foundation of China (NSFC) (31770989 to Y.W.; 81671520 to Q.C.) and the Shanghai Ninth People's Hospital Foundation of China (JYLJ030 to Y.W.). The authors have no conflicts of interest to declare. TRIAL REGISTRATION NUMBER: N/A.