Effect of vitrification on clinical outcomes of cleavage-stage embryos with poor quality in human embryo cryopreservation

Objective: To evaluate the effect of morphologic factors on survival rate (SR), pregnancy rate (PR), and implantation rate (IR) of human embryo vitrification following frozen embryo transfer (FET) on day 3 post-ovulation. Methods: Women undergoing FET (n = 921) with embryos cryopreserved by vitrification between 2012 and 2013 were enrolled in this retrospective cohort study. Results: Embryos with >9 blastomeres yielded the highest SR of 100%. Lower SR was observed in embryos with 5 to 6 (57.5%) and 4 blastomeres (41.4%). In terms of blastomere symmetry, the SR of embryos with equally sized blastomeres was significantly higher than that of embryos with unequally sized cells (82.5% vs. 64.6%, P < 0.05). As fragmentation increased, SR decreased from 92.1% to 20.6% (P < 0.05). Significant differences were observed among groups when analyzing PR and IR according to the 3 embryonic parameters before vitrification. Embryos with 13 to 16 blastomeres yielded the highest PR (39.5%) and IR (24.1%). The PR and IR of embryos with blastomeres of equal size were significantly higher than those with unequally sized blastomeres (36.5% vs. 21.7%, 23.7% vs. 12.4%, P < 0.05). After warming, embryos with 13 to 16 blastomeres yielded the highest PR and IR (40.9% and 24.2%, respectively). The PR and IR were observed to grow with an increase in the percentage of intact blastomeres (23.2%-38.2%, 14.2%-23.2%). Conclusions: These results show that vitrification methods do not effectively improve survival outcomes for embryos of poor quality and it is needed to develop a comprehensive vitrification protocol that considers all the practical aspects, including the current limitation regarding cleavage-stage embryos of poor quality.