The journey of an embryo starts as a single cell, which keeps on dividing. It passes through the states of 4 cell embryo, then 6-8 cells embryo to finally reach the stage of Blastocyst. Blastocyst stage is typically achieved by day 5. This is the maximum stage, up to which an embryo can be cultured in vitro(outside human body).
‌Transferring embryos at blastocyst stage rather than cleavage cell stage promises better chances of success! Not all embryos at cleavage stage can reach up to Blastocyst stage. Embryos which are really good grade can only achieve that. That is why there are better chances of implantation at Blastocyst stage. A lot of things also depend on the IVF lab quality. If the laboratory conditions are not maintained the way they should be maintained then Blastulation rate is very low(percentage of total embryos reaching the blastocyst stage)