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I’m wondering if you have any information on mosaic embryos? My husband and I are both doctors, and yet we cannot seem to pin down any clear research on the outcomes of implanting mosaics.
—Anonymous
This is a big topic in the field of infertility with many ongoing studies. When embryos are biopsied for PGT-A testing (genetic testing that looks for chromosomal abnormalities), only a few cells are taken. For example, if you take five cells, it is easy to identify an embryo as chromosomally “normal” (euploid) or “abnormal” (aneuploid) if all five of the cells are normal or all five cells are abnormal. In these instances, we can feel confident that the five cells represent the rest of the embryo.
But when some of the five cells show euploid results and some show aneuploid results, this is called a mosaic. This means the embryo could be euploid or aneuploid or (rarely) could have a mix of euploid and aneuploid cells; we just can’t tell from the PGT-A result.
Because some of these embryos are euploid and could result in a live birth, many clinics will make mosaic embryos available for use. When these embryos are transferred into the uterus, they don’t perform as well as euploid embryos, presumably because some mosaic embryos are actually aneuploid and therefore don’t implant or miscarry. Data shows that mosaic embryos have a lower implantation rate, a twofold higher miscarriage rate, and 10-20% lower clinical pregnancy rates and live birth rates compared to euploid embryos.
But not all mosaic embryos have the same prognosis. Embryos that have a low level of abnormal cells (typically defined as less than 50%) — or segmental mosaics that have only aneuploid segments of chromosomes (as opposed to the entire chromosome) — tend to have higher liver birth rates. In some studies, the live birth rate for these types of mosaics approaches what we see with euploid embryos. On the other hand, embryos with high-level mosaicism (over 50%) — as well as embryos with abnormal results on multiple different chromosomes (often called a complex mosaic) — have reduced live birth rates.
Data on the health of the babies born from mosaic embryos shows that the birth weight, the gestational age at delivery, and the rate of congenital abnormalities are similar to those of babies born after euploid embryo transfer. Additionally, the chance that the mosaic mixture of normal and abnormal cells persists in the fetus or baby is extremely rare, typically quoted at less than 1%. So, if the mosaic embryo implants and continues to a live birth, the outcomes are similar to babies born after euploid transfers.
Before considering the transfer of a mosaic embryo, it is important to speak with your physician and genetic counselor to understand these nuances and any risks associated with it.
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I believe there is some question as to whether five cells taken from the trophoblast are actually representative of the entire embryo. After all, we know “abnormal” embryos can grow into healthy babies.