Morphological and Metabolic Assessment of Oocytes and Embryos

Abstract

The non-invasive assessment of preimplantation embryos has been largely limited to the use of morphology and has become the primary tool of the embryologist for selecting which embryo(s) to replace. Since the early years of in vitro fertilization (IVF) it was noted that embryos cleaving faster and those of better morphological appearance were more likely to lead to a pregnancy. Indeed, Edwards and colleagues noted only a few years after the birth of Louise Brown “that cleavage rates on a certain day and overall embryo morphology were valuable in choosing which embryo to transfer”. In 1986 one of the initial large studies [N = 1,539 embryos] examining the usefulness of embryo morphology was published by Cummins et al. and reported that embryo quality scores were valuable in predicting success. Indeed Cummins et al. calculated an embryo development rating based on the ratio between the time at which embryos were observed at a particular stage after insemination and the time at which they would be expected to reach that stage of a hypothetical “ideal” growth rate with a cell cycle length of 11.9 h. Using this scoring system, “normally” growing embryos scored 100, however the scoring system was evidently never assessed prospectively. The following year a study by Puissant et al. reported the grading of embryos based on the amount of anucleate fragments expelled during early cleavage and on developmental speed. They found that embryos endowed with a high score were more often associated with pregnancy and in particular with the occurrence of multiple pregnancy. Interestingly, they already proposed that in the event of a high score: “It might be warranted to replace only two embryos when these conditions are fulfilled.” Here already, in the 1980s, the simple but important concept was introduced that identifying a better embryo will allow us to transfer fewer embryos.