Embryo vitrification and thawing
For Day 3 embryos, our laboratory procedure of vitrification and warming was the same as the method used for human oocytes as reported by Tong et al. previously (32). For blastocysts, a glass micro-needle was used to collapse the blastocyst before vitrification. The following steps were the same as for the Day 3 embryos.
Embryo quality was graded as 1(good), 2(reasonable), or 3(moderate) according to the number of cells, degree of fragmentation and renewed development of the embryo. This standard was based on the ESHRE Istanbul consensus on embryo assessment (33).