Optimal Timing of Euploid Day 6 Blastocyst Transfer in Frozen HRT Cycles, Day 6 or Day 7 of Progesterone Administration.

The goal of this study is to compare the difference in clinical pregnancy, miscarriage and livebirth rate between day 6 euploid blastocyst transfer on the 6th and the 7th day of progesterone exposure in Hormonal Replacement Therapy (HRT) FET cycles. This prospective \& randomized study will only include euploid day 6 blastocysts. This will be the first prospective study of euploid day 6 blastocysts thereby excluding aneuploidy as a cause of miscarriage and implantation failure. The point of randomization will occur on the day of progesterone commencement.

Traditionally the duration of progesterone exposure before embryo transfer has been considered equal for day 5 and day 6 embryos but this may not be the case and warrants further study. The optimal preparation of the endometrium in frozen embryo transfer (FET) cycles is yet to be determined. Synchronization between the embryonic stage and the endometrial window of implantation (WOI) is crucial and progesterone plays a critical role in the WOI (1). Data on the optimal route of administration, the dose and duration of progesterone supplementation before blastocyst transfer are inconsistent (2,3). In view of the current lack of evidence, this study will be of importance.