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New Tools To Select Mosaic Embryos

Couple who had a healthy baby with mosaic embryos

As fertility clinics use PGT-A more frequently, embryologists have discovered new ways to classify embryos, as well as new ways to identify the most viable ones. PGT-A stands for preimplantation genetic diagnosis for aneuploidy.

At CACRM, we recommend PGT-A for older women and for patients with a history of implantation failure and/or miscarriages.

Typically, most embryos are normal (euploid) or abnormal (aneuploid) embryos. Mosaic embryos, however, represent a third category. They contain two or more cell populations with different chromosomal content composed of both euploid and aneuploid cells.

Mosaic embryos can create healthy babies but using them may result in lower IVF success rates. To improve the results with mosaic embryos, physicians and embryologists are exploring different and innovative methods in order to characterize them better. This is especially important for patients who do not have any euploid embryos.

Professional societies such as the Preimplantation Genetic Diagnosis International Society have proposed a new embryo grading system that considers mosaic grading as an entirely separate category.

Collaborative Research Study Among Fertility Clinics

Reproductive endocrinologists from major fertility centers analyzed a total of 1000 embryo transfers of mosaic blastocysts. They published their findings in a journal titled, Assisted Reproduction.

This study reported that the embryos classified as mosaic have a distinct set of clinical outcomes and should comprise a separate PGT-A category.

Researchers discovered that the proportion of abnormal cells and the specific type of mosaic aneuploidy significantly affected the actual clinical outcome of the embryos. They created a prioritization scheme as well as guidelines for choosing mosaic embryos based on the following findings:

  • Mosaic embryos experienced much less favorable clinical outcomes than euploid embryos. Additionally, mosaic embryos usually had fewer grade A embryos and a higher number of grade C embryos.
  • Chromosome mosaic embryos with level (percent aneuploid cells) <50% (low level) had significantly more favorable outcomes than the >50% (high level).
  • The number of affected chromosomes in mixed mosaicism correlated with significantly poorer outcomes.
  • Maternal age did not affect the outcomes of the mosaic embryos.
  • Combining mosaic level, type, and embryo morphology revealed the order of subcategories regarding the likelihood of a positive outcome.

The prioritization scheme outlined in this particular study can be applied to any embryo by considering its’ mosaic attributes and morphology.

Can Aneuploidy be Overcome? 

The reproductive medicine field is continuously investigating the possibilities of mosaic embryos, as well as the reasons behind why certain mosaic embryos can produce seemingly healthy babies.

The analysis of 1000 mosaic embryo transfers provides statistically significant evidence to best identify the traits of mosaicism that affect implantation and ultimately miscarriages. This analysis offers a blueprint as well as tools for ranking mosaic embryos in the clinic for their optimal clinical management.

If you should have any questions about the decisions that we make regarding embryos to use during your IVF transfer, please discuss this important topic with Dr. Arnold.











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