ABSTRACT
Mosaicism is the presence of two or more chromosomally different cell lines in an organism, embryo, or cell line. While uniform aneuploidies mostly derive from aberrant meiosis, embryo mosaicism can originate from post-zygotic segregation errors in an either euploid or aneuploid conceptus. Mosaicism has been detected by fluorescent in situ hybridization (FISH), array comparative genomic hybridization (aCGH), array single nucleotide polymorphism (aSNP), quantitative polymerase chain reaction (qPCR), or next-generation sequencing (NGS), the latter having increased analytical resolution and diagnostic sensitivity. The 2016 PGD International Society (PGDIS) position statement on PGT-A recommended that diagnoses of embryos with aneuploidy variations 20%–80% should be considered mosaics. However, these criteria do not take into account biological variability or artifacts. True mosaicism rate and detection capabilities in preimplantation embryos have yet to be fully established. Varying assay sensitivity and the lack of convincing data on the developmental potential of mosaic embryos generates decisional uncertainty when considering embryos for transfer. More robust studies on the incidence and developmental fate of mosaic embryos should be performed. Meanwhile, the relevance and potential appropriateness of reporting embryo mosaicism diagnosed from single biopsies need to be thoroughly reassessed in light of emerging findings.
