Chromosomal mosaicism in human preimplantation embryos: a systematic review.

BACKGROUND Although chromosomal mosaicism in human preimplantation embryos has been described for almost two decades, its exact prevalence is still unknown. The prevalence of mosaicism is important in the context of preimplantation genetic screening in which the chromosomal status of an embryo is determined by the analysis of a single cell from that embryo. METHODS Here we report a systematic review and meta-analysis of studies on the chromosomal constitution of human preimplantation embryos. In 36 studies, out of 2117 citations that met our search criteria, data were provided extensively enough to allow classification of each analysed embryo with prespecified criteria for its chromosomal makeup. The main outcome of this classification was the prevalence of chromosomal mosaicism in human preimplantation embryos. RESULTS A total of 815 embryos could be classified. Of these, 177 (22%) were diploid, 599 (73%) were mosaic, of which 480 (59% of the total number of embryos) were diploid-aneuploid mosaic and 119 (14% of the total number of embryos) were aneuploid mosaic, and 39 (5%) contained other numerical chromosomal abnormalities. The distribution of the embryos over these categories was associated with the developmental stage of the embryos, the method used for analysis and the number of chromosomes analysed. CONCLUSIONS Diploid-aneuploid mosaicism is by far the most common chromosomal constitution in spare human preimplantation embryos after IVF. This undermines the reliable determination of the ploidy status of a cleavage-stage embryo based on the analysis of a single cell. Future research should determine the origin and developmental potential of mosaic embryos.