 Although many studies have evaluated the relationship between endometrial thickness and IVF outcome, the results are still controversial. We notice that, during our daily work, for patients undergoing cleavage stage embryo transfer, endometrial thickness seems to have a huge impact on IVF outcome; however, for blastocyst stage embryo transfers, IVF outcome seems to have little association with endometrial thickness. Thus, the aim of this prospective observational study is to explore the relationship between endometrial thickness and IVF outcome in patients undergoing cleavage stage and blastocyst stage embryo transfers, respectively. 