Last year, at the American Society of Reproductive Medicine’s (ASRM) annual meeting, mosaic embryos were a big discussion. Should we transfer them? This year, data was presented on comparing clinical outcomes from transfers of 1,000 mosaic embryos to a control group of 5,561 euploid embryos. What is a mosaic embryo you may ask? When an embryo reaches blastocyst stage, the trophectoderm (or what will become the placenta) is biopsied (~ 5 cells) for chromosomal analysis. Originally, genetic testing results showed euploid (normal number of chromosomes) and aneuploid (abnormal number). Mosaics, which are now often being reported with improving technology, are embryos that have a mixture of cells—meaning that not all the cells from a single embryo tested are the same. Some are normal and some are not. So, what does this mean for the embryo? Can this result in a healthy pregnancy? Is there a higher chance for miscarriage, or even abnormal fetal development?
This oral abstract presented at ASRM attempted to answer these questions. Next Generation Sequencing was used for PGT-A and an embryo was categorized as mosaic if it had 20-80% aneuploidy content (for example 2-4 cells were abnormal). What did they find? Mosaic embryos had a lower rate of implantation (57 vs 47%) and lower rate of ongoing pregnancy (53 vs 37%), which was even lower if the mosaic included the entire chromosome. Rates of miscarriage were 20-25% depending on the type of mosaic compared to 8.6% with a euploid embryo transfer. Furthermore, the more abnormal cells found in the biopsy, the worse the prognosis. Importantly, in those that did implant and result in ongoing pregnancies, prenatal testing did not show matching mosaicism in the fetus as the embryo tested. Meaning that those that continued to grow seemed to have self-corrected. Outcomes were similar in terms of gestational age at birth and birth weight for those that were transferred euploid or mosaic. Dr. Maya Barsky indicates that we have been transferring mosaic embryos before genetic testing was even an option, but having the knowledge now means we have to be responsible with what we do with it. There is a ranking system to transferring genetically tested embryos and we should only even begin to consider mosaics when there are no euploid embryos available. This should be done alongside genetic counselors to help identify which embryos may lead to highest success, but even more importantly healthy pregnancies and babies.
In summary transferring mosaic embryos yields lower pregnancy rates, higher miscarriage rates but if the pregnancy were to continue it is likely a normal pregnancy. However, caution needs to be taken as there is no data on the health of children after delivery into adulthood, and this is only preliminary data. We will be looking forward to seeing the full article published after a peer review.