ABSTRACT
Mosaic embryos are characterized by the presence of chromosomally different cell lines within the same embryo. Until recently, mosaic embryos were not transferred because they were considered abnormal; however, recent studies demonstrate that mosaic embryos composed of euploid and aneuploid cell lines (i.e., euploid/diploid mosaic embryos) hold the potential to implant, and result in the birth of healthy babies. Therefore, the transfer of these embryos is now offered as an option for those patients with no euploid embryos available in some clinics. To date, over 500 transfer of mosaic embryos have been reported. The results of these reports indicate that mosaic embryos miscarry more often and implant less often than euploid embryos, but that approximately 30% of them can still result in a viable pregnancy. So far, mosaic embryos are considered a category in between euploid and fully aneuploid embryos in terms of viability. As such, many experts in the reproductive field recommended that clinicians should prioritize euploid embryos over mosaic embryos for transfer and only consider mosaics for replacement when no euploid embryos are available. This chapter aims to provide the latest scientific results on mosaic embryos. First, a brief illustration on the origin and incidence of mosaicism in preimplantation embryos is given, then data on embryo viability and their clinical outcomes obtained from the latest publications are presented. Finally, an overview of the methodologies currently available and the validation assessment required for accurate detection of chromosomal mosaicism in preimplantation embryos is given.
