To date, only two societies have issued guideline statements regarding mosaic embryos and recommendations for clinical practice. The first guidelines were shared by the Preimplantation Genetic Diagnosis International Society (PGDIS) in July of 2016; when reporting PGT-A results, the guidelines suggested a ‘cut-off point’ at >20% abnormality detected. Lower levels (<20%) would thus be classified as normal (euploid), >80% abnormal (aneuploid), and the range between 20-80% mosaic.11
At this stage, outcomes data from the transfer of known mosaic embryos was very limited and further recommendations were made on the knowledge obtained from reproductive outcomes where fetal or placental mosaicism had been identified. These chromosome-based recommendations suggested de-prioritization of mosaicism involving chromosomes 2, 7, 13, 14, 15, 16, 18, and 21 based upon their impact on live birth and pregnancy outcomes.
When the Congress on Controversies in Preconception, Preimplantation, and Prenatal Genetic Diagnosis (COGEN) issued their statement soon after, knowledge had already evolved to add an additional comment on the impact the level of mosaicism (low degree versus high degree) can have on an embryo.12 More recently, a study published in Reproductive BioMedicine Online considered four parameters to determine a ‘scoring system’ for embryos based on chromosome change, including the risk for fetal involvement, uniparental disomy, miscarriage, and viable aneuploidy. The result was a comprehensive breakdown of which chromosomes and in what configuration (trisomic versus monosomic) to prioritize for transfer.13 While complicated at first glance, such ‘personalized’ recommendations are where the ultimate answer may lay as we learn more and more about outcomes of known mosaic embryo transfers.