Modern embryo culture techniques enable the growth of embryos in the incubator until day 5 or 6. The extended embryo culture period means that after 4 or 5 days the embryo with the highest potential for development can be selected. At this stage, the embryo is known as a blastocyst.
The lengthening of this period makes sense when there is a sufficient number of fertilised egg cells available (in accordance with German Embryo Protection Law, and depending on age and desired number of embryos for transfer, 3 to a maximum of 6), thus allowing for the identification of the embryo with the best potential for development (top quality embryo). It can be assumed that only between 25 and 50% of “day 2 embryos” will become blastocysts.