Clinical Value of Mosaicism Diagnosis on the Trophectoderm Biopsies

Mosaicism within an embryo is defined as the presence of two or more cell populations with different genotypes. Blastocysts classified as mosaic by Preimplamtation Genetic Testing for Aneuploidy (PGT-A) have been reported to implant less and miscarry more frequently than embryos classified as euploid. Because of the unknown impact of mosaicism on embryo development, these embryos are given low priority and are discarded for transfer. However, recent papers on the transfer of human embryos classified by PGT-A as mosaic suggest that embryos with a low fraction of abnormal cells resulting in viable, chromosomally normal ongoing pregnancies, and high-level mosaics resulting in fewer viable pregnancies, but so far none producing mosaic babies. The apparent presence of mosaicism in an embryo is used as a selection criteria for embryo transfer (ET), introducing a strong bias in terms of patient prognosis and embryo quality. Additionally, it is also possible that some embryos are incorrectly classified as "mosaic" due to technical variability derived from the processing of a uniform aneuploid embryo. The aims of this study is to provide evidences about the clinical significance of chromosomal mosaicism in PGT-A cycles by a prospective non-selection based methodology.