![]() Recent studies proposed that mosaic monosomies and mosaic trisomies have similar implantation rates (46% and 47.2%, respectively p>0.05) and ongoing pregnancy rates (36% and 33%, respectively p>0.05). According to the PGDIS guidelines, mosaic trisomies 1, 3, 4, 5, 6, 8, 9, 10, 11, 12, 17, 19, 20, 22, X, and Y are preferred over mosaic trisomies 2, 7, 13, 14, 15, 16, 18, and 21 when mosaic trisomy is being considered for transfer. In addition, some authors did not find a significant difference in pregnancy rates between monosomic and trisomic mosaic embryos. Although PGDIS recommended the transfer of embryos with mosaic monosomies over those with mosaic trisomies in 2016, this statement was updated and removed in 2019. Trisomic mosaicism can occur in live births with chromosomal aneuploidy and is associated with cognitive and physical impairments. Since monosomic cells are less likely to be viable than trisomic cells, most monosomic cells are removed at the post-implantation phase. Most monosomies arise from mitotic errors and most trisomies result from nondisjunction during maternal meiotic errors. showed that low- (20%–30%) or moderate-degree (30%–50%) mosaic embryo transfer yielded similar clinical and neonatal outcomes in a prospective double-blinded non-selection trial. ![]() 46.6% p<0.001) compared to euploid embryos in the NGS profile. A recent prospective study found that embryos with more than 50% mosaicism have a significantly lower implantation rate (24.4% vs. Embryos with low-level mosaicism are more likely to develop into healthy babies than high-level mosaic embryos, whereas high-level mosaic embryos increase the risk of miscarriage. Some studies found that low-level mosaicism was related to improvement in ongoing pregnancy rates, while others did not find statistically significant results. Clinical outcome data related to high- versus low-level mosaicism still show conflicting results. Relevant medical society practice guidelines and recommendations, including the recent PGDIS 2021 guidelines, are summarized in Table 1.Ĭhromosomal mosaicism has been defined as low-level mosaicism if abnormal cells are in the 30%–50% range and high-level mosaicism if abnormal cells are in the 50%–70% range using the NGS validation algorithm. Their study helped to elucidate the problems presented by mosaic transfer and attempted to provide firm conclusions. They also found that the type and level of mosaicism had a significant impact on the embryo transfer outcomes. They confirmed that combined mosaic embryos have significantly lower implantation and pregnancy rates than euploid embryos. formulated a ranking system using outcome data from one thousand mosaic embryo transfers for the prioritization of mosaic embryos in the clinical setting. suggested classifying mosaic embryos into high- (>50%) and low-level (<50%) groups, with preference for transferring single segmental mosaic embryos over other types of mosaicism. published a study on the chorionic villi samples (CVS) and products of conception (POC) after natural pregnancy to provide a practice guideline whereby mosaic embryos could lead to healthy live births. In 2017, the World Congress on Controversies in Preconception, Preimplantation and Prenatal Genetic Diagnosis highlighted the need for PGT-A in IVF practice and updated the PGDIS position statement on recommendations for clinical practice. In 2016, the position statement of the PGDIS recommended priorities for mosaic embryo transfers based on the specific chromosome involved and the level of mosaicism. Therefore, we aimed to provide the latest clinical outcomes following mosaic embryo transfers in PGT-A cycles and a summary of updated practice recommendations. Given the variability in the management of mosaic embryos, it is important for clinicians to have informative genetic counseling resources available when informing their patients of PGT-A results and giving recommendations for mosaic embryo transfer. On the contrary, a recently published prospective non-selection study reported that the risk of clinical error in the diagnosis of uniform aneuploidy by NGS-based PGT-A was exceedingly low (0%–2%), suggesting that PGT-A has high predictive power. The International Do No Harm Group in in vitro fertilization (IVF) argued against the 2019 Preimplantation Genetic Diagnosis International Society (PGDIS) guideline for mosaic embryo transfer on the basis that the interpretation of mosaicism in PGT-A was misleading. However, arguments for and against transferring mosaic embryos still exist.
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