The embryologist Laura Molina from the Ginemed centers answers us:
Evidently they are embryos that do not have optimal characteristics to develop normally. It is not that they are lost due to the procedure or due to incubation or due to the conditions in the laboratory, but it is the embryo itself that, due to any type of metabolic failure or some type of chromosomal genetic alteration that it may have, automatically those embryos will not progress and will stop in their development. They are blockages that are associated with the quality of the embryo itself, which can come from problems that come from the sperm as well as from the ovules. Actually, it is also a form of selection. This long development also allows us to assess the quality of the embryos because if they do not evolve, they would not become pregnant either.