Why PGS does not work as advertised - the difference between genes and chromosomes

Lots of patients know that the commonest cause for IVF failure is a failure of the embryo to implant. We also know that the commonest reason for implantation failure is a genetic defect in the embryo. This is why it seems very tempting to do pre-implantation genetic screening ( PGS) for all IVF patients, so that you can select the genetically normal embryos . This should increase the chances of implantation and reduce IVF failure. After all, what's the point of transferring a genetically abnormal embryo back into the uterus ? It will just fail to implant, or even worse , end up in a miscarriage.

This makes logical sense, but unfortunately , in real life , biology is far more complicated. I think we need to remember that when we're doing PGS , we're really not doing genetic screening; we're really just doing chromosomal screening, so that a better name really would have been pre-implantation chromosome screening or PCS.

Why is this important ? All these tests ( for example, CCS and NGS) only allow us to make sure that the cell has a normal number of chromosomes. This means we can make sure it is euploid, so that we can screen out which embryos are aneuploid ( they have an abnormal number of chromosomes) .  Aneuploidy is an important cause for failure of implantation as well as miscarriages, but we also need to remember that counting the number of chromosomes is not enough if we want to identify genetically normal embryos. After all, there are over 30,000 genes , and there are only 23 pairs of chromosomes. This means that even though an embryo may appear to be normal on a PGS, this does not mean that it is genetically normal. Thus, a lot of PGS normal embryos will have a normal chromosomal complement, but they will still have lethal genetic defects will which prevent them from implanting.

Unfortunately , most patients don't understand this difference. When they end up spending a lot of money in order to transfer a PGS-normal embryo , and then still end up with a failed IVF cycle or with a miscarriage, they're obviously very bitter and angry and feel they've been misled. This can create a lot of dissatisfaction.

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