Since physicians have been performing preimplantation genetic testing for aneuploidy (PGT-A) biopsing several cells from a blastocyst and using more advanced genetic tests, Reproductive Endocrinologists been at a loss for how to deal with embryos with cells with more than one chromosomal arrangement, one normal and others with a variety of abnormal configurations. Mosaic embryos weren’t easily detectable until about two years ago, when the technology for the test improved. They represents about a fifth of embryos in which multiple cells are tested.
A recent article in the Wall Street Journal, “IVF Testing Spurs a Debate Over ‘Mosaic’ Embryos” reports on a committre report from the American Society for Reproductive Medicine published in the journal, “Fertility & Sterility.”
The committee report explains, “Mosaicism refers to two or more cell populations with different chromosomal complements being present within the same embryo. Mosaicism was first identified as a common phenomenon in cleavage-stage embryos, although the exact prevalence of mosaicism in embryos is unknown. Embryonic mosaicism is believed to be a confounder when trying to interpret PGT-A results, as mosaic embryos are currently classified as either aneuploid mosaic or diploid-aneuploid mosaic, the latter of which is influenced more by early embryo-cleavage events when chromosomal segregation occurs. Trophectoderm biopsy, whereby 4-10 cells are removed from the embryo for chromosomal analysis, has provided several advantages over cleavage-stage biopsy, including the purported improved detection of mosaicism. More research is needed to elucidate the mosaicism phenomenon further.
The dilemma is what to do with embryos identified as mosaic. This is an extremely complex technical issue with no easy pat answers. Look to your Reproductive Endocrinologist for advice on how to proceed in your individual case if you have embryos identified as mosaic. One thing for certain, normal embryos will be transfered first and mosaic embryos transfered only there are no normal embryos suitable for transfer.