On the basis of bibliography of the last years, there is a trend toward blastocyst transfer rather than cleavage
embryo transfer. There are several studies that point out that blastocyst transfer could achieves better results [1&]. Culture media is one of the most important factors in in-vitro fertilization (IVF); it can affect live birth rate, pregnancy rate, implantation rate, fertilization rate and the number of good quality embryos. Embryos cultured in vitro may be exposed to constant stress.
Suboptimal culture conditions force the embryo to undergo adaptations, and thus cause lower pregnancy and higher miscarriage rates. Embryo culture can be performed sequentially with change to a medium with a different composition proposed to follow the embryo’s physiological needs, or single-step media with refreshment, without change of the medium composition; or even, single-step media without any refreshment for uninterrupted embryo culture, especially for applications like time-lapse microscopy  or even limiting the embryo evaluation
only to fertilization and blastocyst stage. If blastocyst score can predict implantation likelihood better
than cleavage embryo score, we could avoid disturbing the embryos by checking their morphology on days 2 and 3 and keep stable the culture conditions up to day 5. Then, we could pick the best blastocyst according to their orphology.
However, there are still some questions that should be discussed, the first one could be: should we transfer embryos always at blastocyst stage? And if we do that, we should change how to work and how to culture those embryos. There are some more questions that remain important and should be answered: should we let embryos grow to blastocyst without any morphology evaluation? If yes, how should we do it? Should we move all to a single-step culture medium? Should we do assisted hatching on day 3 embryos that will be helpful in biopsy on blastocyst stage?