Reproductive outcomes of mosaic embryo transfers – rethinking the clinical relevance of contemporary mosaicism reporting in preimplantation genetic testing for aneuploidy

PURPOSE OF REVIEW: Next-generation sequencing-based preimplantation genetic testing for aneuploidy (PGT-A) has enabled the detection of intermediate copy-number signals, leading to widespread clinical classification of embryos as chromosomally 'mosaic'. Mosaic embryo transfer (MET) is now routinely considered in assisted reproduction, yet the clinical relevance of mosaic classification remains uncertain. This review critically evaluates reproductive outcomes following MET in light of embryonic biology, technical constraints of PGT-A, and study design. RECENT FINDINGS: Across the body of evidence, embryos labelled as mosaic retain substantial reproductive potential, with largely reassuring outcomes once implantation occurs. Apparent differences in reproductive efficiency, particularly among high-level mosaic categories, are better explained by study design bias, diagnostic imprecision, and classification artefact than by a clear biological intolerance to mosaicism. When selection bias and post hoc stratification are minimised, mosaic classification contributes little independent information beyond established clinical and embryological factors. The persistent signal observed in a small subset of embryos is increasingly attributable to misclassified constitutional aneuploidy rather than by adverse effects of mitotic mosaicism. SUMMARY: These findings shift the conceptual framework away from mosaicism as a clinically actionable diagnosis and toward a model in which accurate identification of constitutional aneuploidy is the principal determinant of reproductive outcome.