The influence of morphologic grading and COS protocol on the outcomes of Day 5 versus Day 6 single fresh blastocyst transfers: a retrospective analysis of clinical outcomes from one center experience

Introduction The purpose of this study was to evaluate whether the blastocyst morphologic grading and the protocol of controlled ovarian stimulation (COS) would influence pregnancy outcomes, aiming to provide guidance when choosing blastocyst transfer. Methods The clinical data of 612 patients who received single fresh blastocyst transfer for first cycle, as well as the data of 253 patients who had already delivered were analyzed retrospectively. The patients were divided into two groups according to blastocyst formation time (D5 or D6). The following subgroup analyses were performed: (i) the morphologic grading of blastocyst and (ii) the protocol of COS . Results We observed that D5 single embryo transfer (SET) were associated with higher clinical pregnancy rate (CPR, 59.04% vs. 31.73%, P