Strengths and Limitations
The study had some advantages in its design that afforded strong confidence on the results. Firstly, the study was based on a large sample including 6096 FET cycles. Then, the study divided the female age into three groups to decrease the effects of female age on clinical outcomes. Moreover, the study selected first FET cycle and excluded FET cycles with abnormal uterine to ensure the accuracy of data. One limitation of this study was that the blastocyst quality was not graded finely and only divided into two levels, which may have resulted in a difference of blastocyst quality in the same level and affect the pregnancy outcomes of different groups.