Abstract: Objective:To compare the pregnancy and birth outcome of single frozen-thawed blastocyst transfer in patients of different ages.Methods:The data of 2 414 cycles of single frozen-thawed blastocyst transfer were retrospectively analyzed.The clinical pregnancy rate,implantation rate,abortion rate,ectopic pregnancy rate,multiple pregnancy rate,premature delivery rate,low birth weight rate,birth defect rate and neonatal death rate between the fifth day and sixth day of single frozen-thawed blastocyst transfer(group D5 and group D6) were compared in patients less than 35 years old and patients more than or equal to 35 years old.Results:Regardless of age,the clinical pregnancy rate and implantation rate in group D5 were higher than those in group D6(P<0.01).The differences of the abortion rate,multiple pregnancy rate,premature delivery rate,low birth weight rate,birth defect rate and neonatal death rate between group D5 and D6 group were not statistically significant(P>0.05).Among the patients less than 35 years old,the ectopic pregnancy rate in group D5 was higher than that in group D6(P<0.01).Conclusions:Regardless of patients' age,the pregnancy outcome and birth outcome of the day 5 and day 6 single blastocyst transfer are good,and that of the day 5 is better than that of the day 6,and the days singk frozen-thawed blastocyst transfer should be the first choice.