It is always preferred to freeze A and B quality embryos, although it is also possible to vitrify C quality embryos.
However, the decision to freeze the D quality embryos will depend on the center itself. These embryos have low quality, as well as the probability of survival after thawing. In these cases, it is usually decided to leave the embryos in culture until day 5-6 of development to study their evolution.
Read the full article on: How To Choose The Best Embryo? Characteristics & Techniques ( 65).
