Shortened equilibration time can compromise clinical outcomes in human embryo vitrification

Vitrification is an important way to cryopreserve human embryos and the recommended time of embryo exposure to the vitrification solution is 1 min. However, practically speaking, the duration of embryos exposure to equilibration solution can vary from 5 to 15 min. The purpose of this study was to investigate the effect of different equilibration times on the outcomes of frozen-thawed embryo transfer cycles. The data were collected from our medical records from January 2012 to June 2013 and a total of 517 cycles were included. These cycles were divided into four groups according to the equilibration time: (i) 5-6 min; (ii) 7-8 min; (iii) 9-10 min and (iv) 11-12 min. The results show that there were no differences in terms of survival rate and fully intact embryo rate among the four groups. However, lower clinical pregnancy, embryo implantation and live birth rates were observed in the 5-6 min exposure group (54.6%, 31.9% and 48.2%, respectively) compared with the three other groups. The corresponding rates in the 9-10 min group (73.5%, 47.6% and 64.7%) were the highest. This study indicated that different equilibration times influenced the clinical outcomes of human embryo vitrification and vitrification with shortened equilibration time compromised the clinical outcomes. Appropriate prolongation of the equilibrium time would probably improve the clinical outcomes.