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.