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Is My Embryo Normal, Abnormal or Mosaic?

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Options for Embryo Transfer after PGT-A

Frequently, we get questions about aneuploid and mosaic embryos at TRIO, particularly about our policies regarding transfer. We asked Karen Spitzer, our PGT Programs Manager, to explain these terms and outline some of the issues involved in the decisions around embryo transfer.

PGT-A, or Preimplantation Genetic Testing for aneuploidy, takes a very close look at an embryo at the chromosomal level. This kind of testing can determine the chromosomal status of the embryo, that is, whether it is considered normal, abnormal or contains a mixture of cells. The majority of tested embryos will fit into one of the following categories:

Euploid embryo

A Euploid embryo is an embryo that has the correct number of chromosomes and is considered a normal embryo. Euploid or normal embryos are the ones we hope to transfer for our patients.

Aneuploid embryo

An Aneuploid embryo has either extra or missing chromosomes and is considered abnormal. What does abnormal mean? Aneuploid embryos are associated with adverse outcomes which can include failed implantation, pregnancy loss (miscarriage), or a possible livebirth with physical and or intellectual challenges. For these reasons, at TRIO we do not transfer aneuploid embryos.


Mosaicism refers to those embryos that are found to have a mix of normal and abnormal cells in varying degrees within a single embryo. Approximately 15% of all PGT cases are found to be mosaic. These kinds of embryos represent a grey area between euploid embryos and aneuploid embryos. Mosaic embryos are likely due to errors that occur after fertilization. Mosaic embryos are classified as either low level or high level and can involve issues with one or more chromosomes. Mosaicism has always existed in embryos, but it is only in recent years with the advent of PGT-A technology that we can identify it.

Transferring mosaic embryos is an area of controversy within the PGT community. We do know that mosaic embryos that have been transferred are associated with lower implantation rates, higher miscarriage rates and fewer live births; however, they can at times result in live births.

We continue to learn more about mosaics on an ongoing basis. At this time, there are numerous factors involved when we consider how mosaic embryos are ranked, and still more when we consider these kinds of embryos for possible transfer. This type of analysis is a critical part of any discussion between you and your doctor when considering the possible transfer of a mosaic embryo. At TRIO our policy relating to mosaic embryo transfer is continually updated based on current evidence in the literature, and always involves genetic counselling as well as input from the TRIO physician. After counselling, understanding TRIO’s current policies and discussion with the physician, we believe our patients are in a better, more informed position.

If you have any additional questions, feel free to reach out to Karen Spitzer, or speak with your TRIO doctor about the best approach for your personal situation. Please ask your doctor’s office to provide you with Karen’s direct phone number or email.