To blastocyst or not to blastocyst? That is the question

Recent advances in culture media preparations have allowed for cleavage embryos to be developed to the blastocyst stage. Blastocysts are regarded as having increased implantation potential, and two blastocysts are typically transferred, which reduces the occurrence of high order multiple gestations. However, with current techniques, most cleavage embryos do not become blastocysts and it is not clear how many of these embryos would have implanted had they been replaced at the cleavage stage. Furthermore, experience with blastocyst cryopreservation is lacking and the overall benefit of blastocyst culture is unknown, unless we consider the combined pregnancy rates of both fresh and frozen blastocysts.