Strengths and Limitations
Compared with previous studies,14, 15 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 9832 FET cycles. Then, in order to eliminate the effects of the embryo quality on the clinical outcomes in this study, we selected the FET cycles that the two embryos after thawed were good-quality embryos. Moreover, the study divided the female age into three groups to decrease the effects of female age on clinical outcomes. Furthermore, 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 patients’ characteristic of the number of oocytes from the OPU was not mentioned.