Optimal endometrial preparation for frozen embryo transfer cycles: window of implantation and progesterone support.

With significant improvements in cryopreservation technology (vitrification) the number of frozen ET IVF cycles is increasing and may soon surpass in numbers and success rates those of fresh stimulated IVF cycles. Increasing numbers of elective single ETs are also resulting in more frozen embryos (blastocysts) available for subsequent frozen ET cycles. Optimal endometrial preparation and identification of the receptive window for ET in frozen ET cycles thus assumes utmost importance for insuring the best frozen ET outcomes. Reliable data are essential for defining the optimal endometrial preparation protocols with accurate determination of the implantation window in frozen ET cycles.