Mosaic embryos explained: what mosaicism means for embryo selection
By the MyBabyBridge editorial team · · Last reviewed · 2 min read
During in vitro fertilization (IVF), embryos can grow in the lab to the blastocyst stage, around day 5 or 6. A blastocyst has two main parts: the inner cell mass, which becomes the fetus, and the trophectoderm, the outer layer that becomes the placenta.
What mosaicism means
A mosaic embryo contains a mix of cells with the normal number of chromosomes and cells with an abnormal number. This happens when errors occur as cells divide after fertilization.
How mosaicism is detected
Preimplantation genetic testing for aneuploidy (PGT-A) analyzes a small biopsy of trophectoderm cells. The American Society for Reproductive Medicine (ASRM) explains that a mosaic result is not seen directly in individual cells. It is inferred by analyzing DNA pooled from a group of cells. Because only a few outer cells are tested, the result may not reflect the whole embryo, including the inner cell mass. Test artifacts can also produce mosaic-looking results.
Implications for embryo selection
According to ASRM's 2023 committee opinion, some types of mosaic embryos may have lower implantation rates. ASRM also states that there is currently no evidence-based method to predict the risk of an adverse outcome or to rank mosaic embryos for transfer. Transferring embryos with mosaic results is increasingly discussed as an option, especially when no euploid embryos are available, with genetic counselling beforehand.
It is also worth knowing that the UK regulator HFEA rates PGT-A red for improving the chance of having a baby for most patients, while rating it green for reducing the chance of miscarriage.
Questions to ask your clinic
- Do you recommend PGT-A in my situation, and why?
- How does your lab report mosaic results, and what thresholds do you use?
- What happens to embryos with mosaic results? Will they be stored or discarded?
- Would you consider transferring a mosaic embryo, and what counselling would you offer first?
- What prenatal testing would you recommend after transferring a mosaic embryo?