Importance of Blastocyst Morphology in Selection for Transfer

Prolonged cultivation of embryos to the blastocyst stage has become a routine practice in the human in vitro fertilization program (IVF) since 1999, when the first commercial sequential media were developed. The culture systems have been improved many times and today most of the blastocyst culture media enable the embryos to reach the blastocyst stage in more than 50% of cases (Gardner et al., 1998; Kovačič et al., 2004). The advantages of blastocyst culture are in the possibilities for selection of embryos that have an activated genome (Braude et al., 1988), higher predictive values for implantation on the basis of their morphological appearance as compared with earlier embryos (Gardner and Schoolcraft, 1999; Kovačič et al., 2004) and in a reduction in the number of transferred embryos without compromising pregnancy rate (Gardner et al., 2000a). Blastocyst is also a stage that is better synchronized with endometrial receptivity for its implantation (Croxatto et al., 1978; Gardner et al., 2000c). By replacement of embryos in the blastocyst stage, their exposure to hyperstimulated milieu and consequent endometrial contractions, which could be fatal for them, is significantly shortened (Lesny et al., 1998). Blastocysts also contain a larger number of cells than early stage embryos and should therefore have a better possibility of survival of cryopreservation (Veeck, 2003).