Clinical utility of mosaic and segmental aneuploid embryos in assisted reproduction

Preimplantation genetic testing for aneuploidy (PGT-A) increasingly identifies embryos classified as mosaic and segmental aneuploid, yet the biological significance and clinical implications of these intermediate results remain uncertain. This review synthesizes current evidence on their biological origins, technical limitations in detection, reproductive outcomes, and implications for embryo selection and counseling.

Emerging molecular data show that many embryos labeled as 'mosaic' reflect meiotic errors or technical artifacts rather than true embryonic mosaicism. Trophectoderm biopsy poorly represents inner cell mass ploidy for segmental and mosaic abnormalities, with high rates of discordance and nonreproducibility on re-biopsy. Retrospective studies suggest mosaic embryos implant less efficiently than euploid embryos, particularly with high-level or complex findings. However, newer prospective nonselection studies demonstrate that low-level mosaic embryos may have reproductive potential comparable to euploid embryos. Segmental aneuploid embryos, including nonmosaic segmental aneuploid, can also result in live birth. Neonatal outcomes after mosaic transfer appear reassuring, and prenatal confirmation of the embryonic mosaic finding is rare.

PGT-A mosaic and segmental results reflect biological and technical heterogeneity. Rigid interpretation risks unnecessary embryo discard. Individualized counseling, cautious prioritization, and structured prenatal planning are essential while more standardized classification and long-term outcome data evolve.