Current experience concerning mosaic embryos diagnosed during preimplantation genetic screening

The concept of embryos containing multiple cell lines (mosaicism) is not new, but much attention has been paid to this concept recently owing to recent advances in molecular techniques to analyze human embryos. Mosaicism in embryos has been known and reported for some time, originally in early cleavage-stage embryos diagnosed with the use of fluorescence in situ hybridization (FISH). However, the early data have come under attack owing to the limited ability of FISH to reliably detect the actual copy number count of chromosomes as well as potential ascertainment bias of those early studies, which were all performed on already analyzed embryos found to be aneuploid. More recent molecular techniques for analyzing embryos have allowed scientists to really begin to understand mosaic embryos, and to now transfer and follow this class of embryo. Indeed, it could be said that three classes of embryos now exist after preimplantation genetic screening: euploid, aneuploid, and mosaic aneuploid. This paper attempts to bring to light the latest data on mosaic embryos and to understand how clinicians and others will deal with this issue today and in the future. Finally, an attempt is made to look to other fields of genetics to understand how this important issue can be dealt with as a group much better than any one individual group may be able to. (C) 2017 by American Society for Reproductive Medicine.