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Aneuploidy screening may diagnose embryos as euploid (normal, two chromosomes for each pair and 46 in total), aneuploid (abnormal, increased or decreased number of chromosomes) or mosaic.

Can embryos diagnosed as mosaic by Preimplantation Genetic Diagnosis (PGD) be transferred?

December 2018

Aneuploidy screening may diagnose embryos as euploid (normal, two chromosomes for each pair and 46 in total), aneuploid (abnormal, increased or decreased number of chromosomes) or mosaic. If a blastocyst biopsied on the fifth day by trophectoderm biopsy is diagnosed as being mosaic, it indicates the presence of cells with different chromosomal structure within the same embryo. A mosaic diagnosis can only be made by next generation sequencing (NGS). While below the threshold of 20% mosaicism is considered as euploidy, over 80% is considered as aneuploidy. The interval of 20% to 80% is considered as mosaic. In the international scientific publications, the mosaicism rate at the blastocyst period is reported between 4 and 24%. In our center, this rate is 7%. However, the rate of mosaicism is less than 0.5% in the late term pregnancies. The difference may be explained by the transfer of mosaic embryos, which did not result in a pregnancy or resulted in miscarriages or mosaic embryos that correct themselves and returned to normal.

Considering the biopsy technique, if the piece is taken from the red cells of embryo number 1, the embryo will be diagnosed as aneuploid, but if red and green cells are biopsied together, a mosaic diagnosis can be made. On the contrary, if trophectoderm biopsy is only performed from green cells, the embryo will be diagnosed as completely normal, ie. euploid.

Blastocyst-stage’s biopsy (trophectoderm biopsy) steps

The implantation rate of mosaic embryos reported in the literature is 33% (Fiorentino et al., 2015). In our center, this ratio is around 50% due to the low mosaicism rate of mosaic embryos which were transferred. It is also reported that mosaic monosomies should be prioritized for transfer when compared to trisomies. Initially, the transfer of mosaic trisomies of chromosomes 13, 18, 21, X and Y was not recommended. Nowadays, the mosaicism rate has gained more importance than mosaicity of which chromosomes. Mosaic embryos between 20% and 40% are more suitable for transfer. For this reason, we do not transfer embryos showing mosaicism more than 40%. However, although there is still little available data, no syndromic baby born after mosaic embryo transfer has been reported by the international medical community. Also, our center results support these findings. 10 healthy babies were born with the transfer of mosaic embryos in our center. For this reason, the transfer of mosaic embryos is performed more and more frequently in the centers where IVF is applied.

Mosaicism formation in embryo development

In the light of this information, as a result of chromosomal screening, couples treated in our center may request the transfer of their mosaic embryos if they do not have any normal embryos. If a pregnancy is obtained after the transfer of a mosaic embryo, regular gestational controls should not be neglected. It is also highly recommended to perform NIPT (chromosomal screening from the mother’s blood) or amniocentesis if their obstetrician advised so.

NGS profile of trophectoderm cells biopsied from an euploid embryo

NGS profile of trophectoderm cells biopsied from a mosaic embryo (6th chromosome: mosaic monosomy; 7th chromosome: mosaic trisomy).

NGS profile of trophectoderm cells biopsied from a mosaic embryo (18th chromosome: mosaic trisomy).

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