Answer from: Raúl Olivares, MD
PGS stands for Preimplantation Genetic Screening, PGT-A stands for Preimplantation Genetic Testing for Aneuploidies. In both cases, we’re going to do a biopsy of the embryo. We’re going to make a hole in the zona pellucida, which is the sternal layer of the embryo, and we’re going to take some cells of the trophectoderm, which is the structure that later will become the placenta in the fetus.
Usually, we try to get between 3-5 cells in an embryo, which roughly has more than 250 cells. Once we get these cells, we’re going to study them just to make sure that the number of chromosomes is the right one. We’re going to create copies of this material, of this stuff, these chromosomes, and then we’re going to get a report in which we are going to detect if each chromosome has a standard amount of stuff, if we have a higher amount or lower amount.
What does it mean? If we have a standard amount, it’s probably because the embryo has 2 chromosomes. If at some point one of the chromosomes has a higher amount than the standard one, it could be because instead of 2, there are 3 chromosomes, that’s a triploidy. If at some point, the amount of the stuff is lower, that could mean that this embryo has only 1 of those chromosomes having monosomy. With this graphic, we’re going to know if all the chromosomes are at the same level or if there is an excess or lack of stuff at some point, which is going to show us if the embryo has extra chromosomes or fewer chromosomes.
This is currently done by a technology called NGS, which has shown very high accuracy. We currently think that the efficiency of PGS is very similar to amniocentesis, which is the test in which the fetus cells are being studied once the patient is around 14-15 weeks. It is not probably as accurate as amniocentesis because in PGS we only work with 3-5 cells, and in the amniocentesis, we usually have 15-20, or even 50 cells, the more cells you have, the more accurate the test is going to be.
We accept that with NGS, so far, we can offer 99% accuracy, which is very good to confirm that the embryo is genetically normal.