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Background:Despite recent advances that have improved the pregnancy success rates that can be achieved via in vitro fertilization (IVF) therapy,it is not yet clear which blastocyst morphological parameters best predict the outcomes of single blastocyst transfer.In addition,most of the previous studies did not exclude the effect of embryo aneuploidy on blastocysts transfer.Thus,the present study investigated the predictive value of various parameters on the pregnancy outcomes achieved via the transfer of frozen euploid blastocysts.Methods:The study retrospectively analyzed 914 single euploid blastocyst transfer cycles that were performed at the Peking University Third Hospital Reproductive Medical Center between June 2011 and May 2016.The expansion,trophectoderm (TE),and inner cell mass (ICM) quality of the blastocysts were assessed based on blastocyst parameters,and used to differentiate between excellent,good,average,and poor-quality embryos.The relationship between these embryo grades and the achieved pregnancy outcomes was then analyzed via the Chi-square and logistic regression tests.Results:For embryo grades of excellent,good,average and poor,the clinical pregnancy rates were 65.0％,59.3％,50.3％ and 33.3％,respectively;and the live-birth rates were 50.0％,49.7％,42.3％ and 25.0％,respectively.Both the clinical pregnancy rate (x2 =21.28,P =0.001) and live-birth rate (x2 =13.50,P ＜ 0.001) increased with the overall blastocyst grade.Both rates were significantly higher after the transfer of a blastocyst that exhibited either an A-grade or B-grade TE,and similarly,an A-grade ICM,than after the transfer of a blastocyst that exhibited a C-grade TE and/or ICM.The degree of blastocyst expansion had no apparent effect on the clinical pregnancy or live-birth rate.All odds ratio were adjusted for patient age,body mass index,length (years) of infertility history,and infertility type.Conclusions:A higher overall euploid blastocyst quality is shown to correlate most strongly with optimal pregnancy outcomes.The study thus supports the use of the described TE and ICM morphological grades to augment current embryo selection criteria.