Diagnosis and Clinical Management of Embryonic Mosaicism

Embryonic mosaicism occurs when two or more cell populations with different genotypes are present within the same embryo. New diagnostic techniques for preimplantation genetic screening (PGS), such as next-generation sequencing, have led to increased reporting of mosaicism. The interpretation of mosaicism is complicated because the transfer of some mosaic embryos has resulted in live births. Mosaic embryos may represent a third category between normal (euploidy) and abnormal (aneuploidy). This category of mosaic embryos may be characterized by decreased implantation and pregnancy potential as well as increased risk of genetic abnormalities and adverse pregnancy outcomes. Euploid embryos should be preferentially transferred over mosaic embryos. Genetic counseling is necessary before the transfer of a mosaic embryo is considered. Certain types of mosaic embryos should be preferentially transferred over others. Transfer of embryos with mosaic trisomies 2, 7, 13, 14, 15, 16, 18, and 21 may pose the most risk of having a child affected with a trisomy syndrome; however, the transfer of embryos with mosaic monosomies or other mosaic trisomies are not devoid of risk. Patients must be counseled about the risk of undetected monosomies or trisomies within a biopsy specimen as well as the risk of intrauterine fetal demise or uniparental disomy with the transfer of mosaic embryos. Until more data are available, patients should be encouraged to undergo another cycle to obtain euploid embryos, when possible, rather than transferring a mosaic embryo.