Embryos are transferred at either the cleavage-stage (Day 2 or 3) or the blastocyst-stage (Day 5 or 6). The particular stage at which the embryo is transferred depends on a number of factors. Two of the main determinants are how many embryos are available and what their quality looks like. Because the lab is a stressful environment for the embryo, weaker embryos are transferred early rather than late, that is, by Day 2/3 rather than Day 5/6. The reason for this is that weak embryos that still have a chance of making a baby within the womb might succumb if left too long in laboratory conditions.