Frequency and clinical management of numerical and segmental mosaicism in embryo biopsies obtained for preimplantation genetic testing (PGT)

Preimplantation genetic testing (PGT) for aneuploidy is an established indication to increase the pregnancy rate while still transferring a single embryo. Trophectoderm biopsy in experienced hands is safe with no association with birth defects. Accuracy is high using next-generation sequencing and clinical utility proven; predictive value of achieving pregnancy when transferring a euploid embryo is 10-fold greater than when transferring an aneuploid embryo. In our unit next-generation sequencing results show 8% of embryos having numerical mosaicis and 14% having segmental mosaicism. Mosaicism is defined as 20%–80% aneuploid DNA, compared with <20% in a sample considered nonmosaic euploid. Transfer of a mosaic (20%–80% aneuploid DNA) embryo may be appropriate if there are no euploid embryos in the cohort of a given cycle. Guidelines exist for prioritization of mosaic embryos for transfer.