Day 2 embryos are in the 4-cell period and day 3 embryos are in the 8-cell period. Day 3 embryos can be used for transfer or to continue culturing blastocysts, while day 2 embryos are more unstable and are usually used to continue culturing.
The main difference between 2-day and 3-day embryos is in the number of cells, and 3-day embryos are more stable and more likely to transfer successfully than 2-day embryos.
A 2-day embryo is 4-cells and a 3-day embryo is 8-cells. 3-day embryos can be used for assisted IVF transfer. 3-day embryos can be graded for quality, in which Grade 1, Grade 2, and Grade 3 3-day embryos can be used for in vitro fresh embryo transfer, and if they are not transferred, they can be frozen, or cultured into blastocysts before transfer or freezing.