A mosaic embryo contains both chromosomally normal (euploid) and abnormal (aneuploid) cells in the same embryo. This is identified through PGT-A (Preimplantation Genetic Testing for Aneuploidy).
Levels of Mosaicism
- Low-level mosaic — 20–40% abnormal cells
- High-level mosaic — 40–80% abnormal cells
- > 80% — considered aneuploid
- < 20% — considered euploid
Should You Transfer a Mosaic Embryo?
Priority order: Transfer euploid embryos first. If none available, consider mosaic embryos — prefer low-level over high-level, and avoid chromosomes associated with viable trisomy (13, 18, 21) or monosomy X.
Evidence Summary (PGDIS 2019, ESHRE 2020)
- Mosaic embryos have ~20% lower pregnancy rate vs euploid
- Higher miscarriage rate
- Most live births from mosaic transfers have normal chromosomes (self-correction in early embryo)
- No increased congenital abnormality reported vs euploid transfer
Factors to Consider
- Maternal age (older patients have fewer euploid options)
- Number of remaining euploid embryos
- Mosaicism level and specific chromosomes involved
- Couple's readiness and understanding
- Genetic counseling essential
Prenatal Confirmation
If transferring mosaic, recommend NIPT + amniocentesis during pregnancy to confirm fetal chromosomes.
