Important Information If You’re Doing PGT-A: Mosaic Embryos

Mosaic embryos are the talk of the fertility town and the focus of this post.

If you’re making the decision whether to do PGT-A on your embryo(s), then read this post about considerations because it’s a nuanced decision that is unique to you (and your partner). What’s missing in my previous post are mosaics.

There’s always more complexity when it comes to fertility, isn’t there? But this knowledge could impact your dream of having a baby so it’s important to add this as part 2.

So why now? Part of this post is from recent studies including the first prospective study (Capalbo et al, 2021), as opposed to previous retrospective ones (like Viotti et al, 2021) and finding that mosaics are misunderstood. It’s also in part from findings from a 2022 published paper (edited December 2022; Bardos et al, 2022) comparing IVF labs and revealing that the lab the PGT-A process is completed makes significant difference. I’m not sharing any information to persuade you, but to assist you to make an informed decision. It’s important to do what’s right for you.

Personally, I chose to PGT-A my embryos and did end up with a mosaic. I only got to understand more about mosaics long after I received my report from my clinic. I’m still learning as a coach and health researcher for my clients and other people I serve, like you, but not as a fertility genetic counsellor. I look to them for their expertise which is included here. Let’s get into it.

Mosaic embryos have a chance at becoming babies for keeps

If you’re having an egg retrieval(s) then you’re hopefully aware that if you have a few (day 5) embryos from your cycle(s), then on average about 60% will be euploid (chromosomally normal) but that percent decreases with the age of the ovaries, ie age of the person having the eggs extracted. That’s the nature of this type of assisted reproduction. However, PGT-A isn’t just abnormal verson normal or aneuploid vs euploid. There’s the grey area in-between. There could be mosaics. Mosaics have a chance at success, although all mosaics aren’t the same. Some mosaics can self correct and all have the potential to. There seems to be controversy around whether aneuploid embryos self correct or rather that there could be a false positive, but what is more clear is that some mosaics do have this potential for success. If there are enough normal cells in a mosaic then the abnormal parts resolve, they could continue on to become a baby for keeps. If there were too many abnormal cells then they would not.

There are different types of mosaics and they have varying chances of success.

All types have lead to a live birth in the history of embryo transfers, but the chance of success vary depending on the type. Low and high level segmental mosaics have the highest chance of success (but still not as likely as a euploid) even above low level mosaics – where 20 to 30% of the sample is aneuploid or abnormal. They have a much better chance than high levels for success. Not all mosaics are categorized as the same or have the same deletion, but all do have potential for becoming a baby for keeps.

It’s possible that an embryo was labelled aneuploid but was actually mosaic.

This might be helpful in your decision making and an important question to ask your clinic. An error can happen in rare cases but it happens and it’s possible your embryo was wrongfully labelled an aneuploid but it was a mosaic. Some clinics don’t even distinguish between aneuploid versus a mosaic even though a mosaic does have a chance at success. This is in part because PGT-A is used to help select which embryo to transfer. Labs can differ in how they label an embryo as mosaic, not the typical <20% aneuploid cells as euploid and >80% are aneuploid but instead might establish the threshold as 30% or less but it’s not certain. Ask your clinic.

Mosaics aren’t more common with age.

Unlike aneuploid (abnormal) embryos, mosaics happen at any age and don’t increase in likelihood by age. The older or younger you are (as the egg supplier) doesn’t mean the more or less likely you’ll have a mosaic. They just happen.

Ask your clinic to label any mosaics if you do PGT-A.

Ask your clinic to show you the report and breakdown of the mosaics and types and deletions. Have them explain it to you. Get all this information so you can be informed. Again, like PGT-A in general, PGT-A is about deciding which to transfer next and mosaics are part of this decision.

Ask a genetic counsellor for their input.

Talk to your genetic counsellor to get the best, most personalized information before deciding and, if applicable, also once you have the report. Ask yours at your clinic or if you don’t have one or if want a second opinion, try the National Society of Genetic Counselors in Canada and more information at Resolve in America. Specifically finding a fertility genetic counsellor is challenging but they have a unique, expert perspective.

Join a Facebook support group.

If you have a mosaic or are considering PGT-A and want to talk to people with lived experience with their mosaic embryo(s) check this Facebook support My Perfect Mosaic Embryo. I have a mosaic embryo and I’m happy to discuss my personal experience but this is a large group so you can get a diversity of experiences.

If you need support I can coach you through it, just book a call. I wish you all the success in the world on your fertility journey, with all the support you deserve.

Here’s a great video with a Q & A from a genetic counsellor who gives a wonderfully succinct explanation of all of this as well.


Bardos J, Kwal J, Caswell W, Jahandideh S, Stratton M, Tucker M, Decherney A, Devine K, Hill M, O’Brien JE. Reproductive genetics laboratory may impact euploid blastocyst and live birth rates: a comparison of 4 national laboratories’ PGT-A results from vitrified donor oocytes. Fertil Steril. 2022 Nov 29:S0015-0282(22)01897-0. doi: 10.1016/j.fertnstert.2022.10.010. Epub ahead of print. PMID: 36460523.

Capalbo A, Poli M, Rienzi L, Girardi L, Patassini C, Fabiani M, Cimadomo D, Benini F, Farcomeni A, Cuzzi J, Rubio C, Albani E, Sacchi L, Vaiarelli A, Figliuzzi M, Findikli N, Coban O, Boynukalin FK, Vogel I, Hoffmann E, Livi C, Levi-Setti PE, Ubaldi FM, Simón C. Mosaic human preimplantation embryos and their developmental potential in a prospective, non-selection clinical trial. Am J Hum Genet. 2021 Dec 2;108(12):2238-2247. doi: 10.1016/j.ajhg.2021.11.002. Epub 2021 Nov 18. PMID: 34798051; PMCID: PMC8715143.

Popovic M, Dhaenens L, Taelman J, Dheedene A, Bialecka M, De Sutter P, Chuva de Sousa Lopes SM, Menten B, Heindryckx B. Extended in vitro culture of human embryos demonstrates the complex nature of diagnosing chromosomal mosaicism from a single trophectoderm biopsy. Hum Reprod. 2019 Apr 1;34(4):758-769. doi: 10.1093/humrep/dez012. PMID: 30838420.

The Preimplantation Genetic Diagnosis International Society position statement on the transfer of mosaic embryos 2021

Viotti, M., Victor, A.R., Barnes, F.L., Zouves, C.G., Besser, A.G., Grifo, J.A., Cheng, E.H., Lee, M.S., Horcajadas, J.A., Corti, L. and Fiorentino, F., 2021. Using outcome data from one thousand mosaic embryo transfers to formulate an embryo ranking system for clinical use. Fertility and Sterility115(5), pp.1212-1224.

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