It tells you a lot about the number and structure of chromosomes from the biopsied samples, from the preimplantation embryo. The embryos grown in the incubators during the IVF process are taken out and biopsied, which is the aspiration of some cells at the blastocysts stage on day 5 of their development. We take those samples and analyse the chromosomes and the structures of the chromosomes in them. The number and structure should be 46 for humans, no more or less than that and without changes in the look of their structure.
At the moment, PGS is a term that actually doesn’t exist anymore because it was originally for Preimplantation Genetic Screening. As technology has changed the IVF world so much, we have to talk about Preimplantation Genetic Testing (PGT). Specifically for aneuploidy that will tell us if an embryo is viable because it contains the correct number of chromosomes. If we have one missing chromosome, the human embryo will never implant. If we have one extra chromosome, sometimes they implant, but it leads to a miscarriage or produces an unhealthy individual.
The test is designed not to make better embryos but not to use those embryos that cannot produce a healthy baby. The concept of PGT for aneuploidy is to shorten the time it takes to become pregnant for any couple looking to have a baby.
With the PGS, we currently gather 3 different pieces of information about the embryo quality depending on the indication for the PGS. The first one is that we can confirm that the embryo has a normal amount of chromosomes with no structural abnormalities. This is the information we get from the PGT-A, we can also confirm that in cases in which the patients are known healthy carriers of mutations that can cause diseases in the baby like cystic fibrosis or thalassemia.
We can also confirm that the embryos are healthy carriers or free of the disease discarding those embryos that are going to be affected by this single gene-disease. This is done by the PGT-M, and we can also confirm that the embryos are healthy in cases in which patients have translocations, which means that they have exchanged parts of the chromosomes that can be balanced or imbalanced. If they are imbalanced, it can cause problems with the babies.
With the PGT-SR, we can confirm that the embryos are healthy or that they carry this completely balanced translocation, and it’s not going to affect the health of the baby.
PGD can help in a situation where you have genetic problems in your family, or your age is advanced, also if the sperm quality is poor. PGD will give you more information about your possible embryo. If you had a miscarriage more than once before or in situations when you have IVF procedures once or twice before, and you haven’t achieved a pregnancy in the end, PGD can help as well.
