Firstly, we need to be clear about what is genetic testing of the embryos and what it intended to do.
There are different types of genetic tests done on eggs, embryos, and embryo by products. The commonest reason for testing the embryo, is to ensure that it has a correct set of chromosomes before it is placed back into the womb in an ivf cycle, this is known as PGT -A (preimplantation genetic testing – aneuploidy. Formerly known as PGS – preimplanatation genetic screening). – chromosomes by the way, are like the books that codes your life story – imagine there were 23 volumes bound as 1a 1b, 2a 2b – of which each parent contribute to 50%. Another test that is often done, is for ‘monogenic’ disorders (PGT -M (monogenic) – commonest of which would be cystic fibroisis. The genetic testing done here, identifies a single gene – to exclude a known genetic condition that could be passed on from the parents. In this case – a gene is as small as a sentence in the 23 volumes of your book. And lastly, very rarely, some patients have a switch in their chromosomes – imagine the latter half of book 13, was now attached onto the book 18 and whilst the latter of 18 was attached onto 13. This is tested by ‘PGT’ – SR.
Whilst it is possible to do genetic testing on eggs, day 3, and embryo breakdown products, the universally accepted test is the day 5 PGT-A testing of the embryonic placental cells (trophectoderm).
The reason why such a test was designed was to overcome the not so perfect success rates with IVF. More specifically, it is known that approx. 50% eggs – and so 50% embryos made even in a young woman in her 20’s is going to be chromosomally abnormal. This rate will rise to a staggering 80-90% in a woman in her early 40’s. Now these embryos would usually not implant, and even if they do, most go on to miscarry. Downs syndrome is actually one such chromosomal disorder that would be an example of such a slip up by nature, that keeps on growing until birth. Most downs syndrome embryos will not implant, or miscarry.
In other words, even in a young well woman in her 20’s, the best clinical pregnancy rate (pregnancy with a heart beat detected) by an embryo transferred would be approx. 50%, and a woman in her early 40’s would be around 20%’s. A genetically tested normal embryo is known to be reported up to 60-70% clinical pregnancy rate. So far so good, – we should test everyone in that case right? In that case we could improve pregnancy rates, reduce miscarriage rates, and prevent all chromosome abnormalities?
Well here is the catch!
Firstly, it is important to understand that IVF is an imperfect process. It is commonly misconceived that every woman with the right dose of medications, will always be able to get the remaining 10 out of 10000 eggs anytime the fertility specialist administers the drugs. And thereafter, all eggs will fertilise, and all those fertilised will go on to give us 10 embryos each time – what a wonderful world that would be. Unfortunately the reality is far from that. Indeed less than half of the women treated (not even counting the number of cycles) – even had an embryo to freeze. – that is, 50% of women going through IVF, either had no embryos to transfer, – or only one embryo transferred from the fresh IVF cycle. Whats the point in testing a single embryo? – If it was normal – it probably would have fared better to be transferred than to be frozen (PGT-A requires embryo freezing) and go through invasive testing.