Abstract: Objective To analyze blastocyst development stage and the effects of its inner cell mass and trophoblast quality on pregnancy outcome, so as to provide help for the timing of blastocyst freezing and the selection of thaw embryos during frozen-thawed embryo transfer（FET） cycle. 
Methods This study retrospectively analyzed the data of patients who received the single blastocyst FET cycle in the Department of Reproductive Medicine,Maternal and Child Health Hospital of Baoding City. According to blastocyst expansion/hatch status and blastocyst quality, blastocysts were divided into A1 group(stage 4 high-quality blastocyst group),A2 group(stage 4 non-high-quality blastocyst group),B1 group(stage 5 high-quality blastocyst group),B2 group(stage 5 non-high-quality blastocyst group),C1 group(stage 6 high-quality blastocyst), and C2 group(stage 6 non-high-quality blastocyst).The difference in pregnancy outcome of FET cycle of blastocysts of the same quality at different developmental stages, and the difference in pregnancy outcome of FET cycle of blastocysts of different quality at the same developmental stage were compared. 
Results Compared with the high-quality blastocyst groups, the pregnancy rate and implantation rate of the FET cycle in the A1 group were significantly higher than those in the B1 group(P＜0.05), while there was no significant difference in the pregnancy rate and implantation rate of the FET cycle between the A1 group and the C1 group(P＞0.05). However,the pregnancy rate and implantation rate of the C1 group showed a downward trend,and there was no statistical difference in parameters such as abortion rate,live birth rate, male/female ratio, gestational age, and newborn weight during the FET cycle among the three groups(P＞0.05). Compared with non-high-quality blastocyst groups, there was no statistical difference in pregnancy rate, implantation rate, miscarriage rate, live birth rate, male/female ratio, gestational age and newborn weight during FET cycle in A2, B2, and C2 groups(P＞0.05). Compared with theblastocyst groups of different qualityat the same developmental stage, the pregnancy rate, implantation rate, and live birth rate of A1 group were significantly higher than those of A2 group(P＜0.05), but there was no statistics between the two groups with respect to miscarriage rate,male/female ratio,gestational age and newborn weight(P＞0.05); There were no significant differences in pregnancy rate, implantation rate, miscarriage rate,live birth rate, male/female ratio,gestational age, and newborn weight during FET cycle in group B1, as compared with group B2, and in group C1, as compared with group C2(P＞0.05). 
Conclusion The developmental stage of the blastocyst has a certain impact on the outcome of the blastocyst following FET. The pregnancy rate and implantation rate of stage 4 high-quality blastocyst after FET are significantly better than clinical outcome of stage 5 high-quality blastocyst after FET,which, however, are not significantly different from those of stage 6 high-quality blastocyst during FET cycle. However, the pregnancy rate and implantation rate of stage 6 blastocysts have a downward trend. The clinical outcome of stage 4 high-quality blastocyst during FET at the same developmental stage is significantly higher than pregnancy rate and implantation rate of stage 4 non-high-quality blastocyst during FET. Therefore,as for the blastocyst FET cycle, we should give priority to stage 4 high-quality blastocyst.

Key words: embryo transfer, blastocyst staging, pregnancy outcome