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So you’ve decided to do pre-implantation genetic testing for aneuploidy (PGT-A) on your embryos during IVF, and now that you’ve gotten the results back, you find out some of your embryos are mosaic. You’ve heard of “normal” and “abnormal” embryos before, but what is a mosaic embryo? Can mosaics be transferred and still result in a live birth?
We spoke with Dr. Michael Guarnaccia of Pinnacle Fertility in New York City to learn more.
What is a mosaic embryo?
A euploid embryo is normal, meaning all of the cells in the embryo have the right number of chromosomes (46). An aneuploid embryo is abnormal, meaning all of its cells have a different number of chromosomes.
According to Dr. Guarnaccia, a mosaic embryo — defined as an embryo that contains more than one “chromosomally distinct” cell line — falls somewhere in the middle. Basically, it has a mix of euploid and aneuploid cells. Mosaic embryos are pretty common, so you are certainly not alone if you are faced with the decision of whether or not to transfer one!
Why the different types of mosaic embryos matter
The first step in deciding whether or not transferring a mosaic embryo is right for you is to understand the different types of mosaic embryos.
One common way to categorize mosaic embryos is by the percentage of aneuploid cells present. Most PGT-A testing labs consider a “low-level mosaic” as having 20-40% aneuploid cells, while a “high-level mosaic” has 40-80% aneuploid cells.
The other way mosaic embryos are categorized is by their type of chromosomal abnormality. Some mosaic embryos have aneuploid cells with whole chromosomes affected, while others have only a segment of a chromosome affected.
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Since they're a mix of normal and abnormal cells, mosaic embryos are somewhere in between euploid and aneuploid in their potential for creating a pregnancy.
So, should you transfer a mosaic embryo?
Dr. Guarnaccia explains, “The potential implications of transferring a mosaic embryo based on available studies have shown that mosaic embryos have lower implantation rates and a higher risk of spontaneous miscarriage.”
One large study compared live birth rates after transferring euploid embryos when compared to 1,000 mosaic embryos. Overall, live birth rates for mosaic embryos ranged from 20.8% to 37.0%, while euploid embryos had a 52.3% live birth rate.
“That being said,” Dr. Guarnaccia says, “it has been shown that the percentage of mosaic cells within the embryo will affect outcomes in terms of probability of pregnancy, risk of non-pregnancy, and risk of spontaneous miscarriage.”
In other words, the best-performing mosaics are low-level and affect only one chromosome, while high-level mosaics affecting more than three chromosomes result in lower live birth rates.
Based on the Preimplantation Genetic Diagnosis International Society’s guidelines, low-level mosaic embryos should be given priority over high-level mosaics for transfer and segmental mosaic embryos may be given priority over whole chromosome mosaic embryos, with the former (segmental) being more likely to represent a false-positive test result due to test artifact than the latter (whole chromosome).
Mosaic embryos and birth defects
You may also be wondering, "What about birth defects resulting from a mosaic embryo transfer?”
Dr. Guarnaccia assures, “A 2021 review of 25 published studies showed that less than 1% of the 2,759 [mosaic] embryos transferred gave rise to an abnormal (aneuploid) pregnancy related to the original PGT-A result.”
Because of this, the PGDIS guidelines don’t currently consider the chromosome that’s affected in the mosaic embryo when prioritizing mosaic transfers.
Transferring a mosaic embryo can be nerve-racking, and the concerns that the embryo will not implant, miscarry, or that it will develop into a baby with a genetic disorder are very valid ones.
That’s why it’s so important to have an honest conversation with your fertility specialist, as well as with a seasoned genetic counselor, who can advise you on the risks of embryo transfer based on the number of chromosomes involved in the mosaic result. To help you feel more confident, Dr. Guarnaccia also recommends prenatal testing for all pregnancies involving mosaic embryos.
If you take one thing away from this article, know that if transferring a mosaic embryo is your only option during IVF, it can result in a healthy baby, and there is hope. You’ve got this!
Sean Lauber aka "Embryoman" is a former embryologist and creator of Remembryo.com, where he provides weekly summaries of the latest IVF research. You can also follow him on Facebook, Instagram and TikTok.