This is very fortunate for us because these cells contain a treasure trove of information. The most common thing IVF clinics look for is aneuploidy, which is a medical term meaning “this embryo has an abnormal number of chromosomes”. The term for this type of testing is “PGT-A”, and it’s performed in roughly half of all IVF cycles in the US today.
Human embryos with the wrong number of chromosomes are surprisingly common, both among IVF patients and natural births. But this wasn’t very well understood before the first use of pre-implantation genetic testing in the late 1980s.
I should note, in many cases embryos with aneuploid cells in the trophectoderm actually have a mixture of aneuploid and euploid cells, and the euploid cells can successfully grow into a healthy embryo. So an aneuploid trophectoderm biopsy doesn’t necessarily mean the embryo is not viable. (Although it does provide some evidence for that.)
Yes, I thought about mentioning mosaicism but it’s such a can of worms with so much debate in the PGT research field that I just left it out.
My reading of the research suggests that mosaicism is far less common than the average PGT test would suggest. Here’s a well done study from 2021 in which the authors took apart 942 embryos and found the true rate of mosaicism to be 3%.
Most PGT labs diagnose mosaicism at far higher rates: 5-15% at most labs. My impression is this is mostly an artefact of noisy, low-density NGS sequencing technology and/or contaminated samples.
It’s also worth noting that we have a single digit number of examples of “mosaicism” in actual people, which suggests to me that there’s either differential apoptosis of aneuploid cells during embryonic development, or that mosaicism is almost always lethal.
I should note, in many cases embryos with aneuploid cells in the trophectoderm actually have a mixture of aneuploid and euploid cells, and the euploid cells can successfully grow into a healthy embryo. So an aneuploid trophectoderm biopsy doesn’t necessarily mean the embryo is not viable. (Although it does provide some evidence for that.)
Yes, I thought about mentioning mosaicism but it’s such a can of worms with so much debate in the PGT research field that I just left it out.
My reading of the research suggests that mosaicism is far less common than the average PGT test would suggest. Here’s a well done study from 2021 in which the authors took apart 942 embryos and found the true rate of mosaicism to be 3%.
Most PGT labs diagnose mosaicism at far higher rates: 5-15% at most labs. My impression is this is mostly an artefact of noisy, low-density NGS sequencing technology and/or contaminated samples.
It’s also worth noting that we have a single digit number of examples of “mosaicism” in actual people, which suggests to me that there’s either differential apoptosis of aneuploid cells during embryonic development, or that mosaicism is almost always lethal.