Question 1

What is mosaicism?

Accepted Answer

A mosaic embryo is an embryo with both normal (euploid) and abnormal (aneuploid) cells. The normal, euploid cells have the correct number of chromosomes (46 chromosomes) while the abnormal, aneuploid cells have extra or missing chromosomes.
The mosaic level refers to the inferred percentage of aneuploid cells in the TE biopsy specimen. Typically, they are reported as either "high" or "low"
The mosaic type refers to the nature of chromosomal abnormality. 'We typically consider the following types of mosaicism: segmental, one chromosome, two chromosome, and complex.

Question 2

How is mosaicism measured? What are the relevant procedures?

Accepted Answer

Preimplantation Genetic Testing for Aneuploidy (PGT-A) evaluates the chromosomal content of a multicellular trophectoderm (TE) biopsy collected from the embryo at the blastocyst stage. The trophectoderm forms the outer layer of the embryo and serves as the cellular precursor to the placenta. If the PGT-A results suggest a mix of cells with euploid and aneuploid chromosomal copy number, it indicates the presence of mosaicism.

Question 3

How does it affect pregnancy outcomes? What does it mean for the baby?

Accepted Answer

IRMET data indicates that features of mosaicism detected with PGT-A can correlate with different clinical outcomes. Genetic counsellors can use the mosaic embryo ranking tool located in the "For Professionals" page to rank mosaic embryos by their likelihood of positive outcome, according to the existing data in the IRMET database. Note that mosaicism also interacts with other parameters to affect pregnancy outcomes. Please refer to the publications by IRMET regarding the babies of mosaic embryo transfers, which, on average, might be indistinguishable from babies of euploid embryo transfers.

Question 4

What next steps can I take if my embryo is mosaic?

Accepted Answer

Consider discussing the results with your clinical provider. Follow this link to find a genetic counselor (Canada and USA only). A genetic counselor will guide you through prenatal/postnatal testing and discuss next steps.

FAQ

What is mosaicism?

How is mosaicism measured? What are the relevant procedures?

How does it affect pregnancy outcomes? What does it mean for the baby?

What next steps can I take if my embryo is mosaic?

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