Zygotes or embryos that reach developmental milestones first (i.e., syngamy, early cleavage, early compaction and early blastulation) are generally believed to have increased chances of normal developmental potential. In recent artificial frozen embryo transfers (FET) in which blastocysts vitrified on day 4 of culture and transferred on P+4, with blastocysts traditionally transferred on P+5, a high rate of implantation has been observed. In addition, a greater number of these implantations resulted in clinical pregnancies (i.e., a conceptus with normal fetal heart activity). In this prospective randomised control study, the investigators will investigate whether this increased viability, is due either to embryo quality, endometrial receptivity, or a combination of the two factors by randomizing the transfer of day 5 blastocysts to P+4 or P+5.