A mosaic embryo is an embryo that has a mix of normal and abnormal cells in different proportions. This condition arises during early development when some cells in the embryo have the correct number of chromosomes (euploid), while others have an abnormal number of chromosomes (aneuploid). Mosaicism occurs naturally when there is an error in the division of cells in the early stages of embryo development. Embryos that are mosaic can have different proportions of normal and abnormal cells. A low-level mosaic embryo would have mostly normal cells and a lower percentage of abnormal cells, while a high-level mosaic embryo would have mostly abnormal cells and a lower percentage of normal cells.

Mosaicism happens after fertilization, which means that it does not increase with the parents ages and occurs equally in all age groups. Preimplantation genetic testing for aneuploidy (PGT-A) is a process that allows for the identification of genetic abnormalities in embryos prior to their transfer in IVF treatment. PGT-A can identify mosaic embryos, but it is not an exact science, and it is only a tool to help patients and physicians choose which embryos to transfer.

Although it is widely accepted that mosaic embryo transfer is associated with a reduced chance of implantation and an increased risk of miscarriage, recent studies have shown that mosaic embryos can lead to healthy pregnancies and live births. A recently published study of 1,000 mosaic embryo transfers found that those that progressed into a late-term pregnancy and full-term birth had similar odds of being born without any discernible genetic differences to a normal embryo. However, transferring a mosaic embryo carries a higher risk of implantation failure, miscarriage, or birth defects. Therefore, an important discussion between the patient and the doctor is whether mosaic and abnormal embryos are safe to transfer.