Embryos are incubated in the IVF laboratory for 5-6 days until they reach the blastocyst stage. Culturing embryos in the laboratory allows selection of the most viable embryos. Blastocysts are assigned a single numerical (1-4) and two letter (A-D) grades. The numerical grade denotes the expansion of the embryo; the first letter – the grade of the cells that become the fetus; and the second letter – the grade of the cells that become the placenta. The letter grades are the most important as they represent the cells that will hatch from the eggshell and lead to a pregnancy. In general embryos graded BC and above are considered viable and frozen for future transfer.
Transfer of autologous frozen blastocysts yield higher births rates that fresh blastocyst transfer.