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Preimplantation Genetic Screening (PGS): Mechanisms & Limitations

The Center for Human Reproduction’s Dr. David H. Barad explains the limitation of Preimplantation Genetic Screening (PGS). PGS often give false-positives because some cells of one embryo can be abnormal when others are normal.

The promise of PGD was that we could find the perfect embryo among an array of embryos – that when looking at them, you couldn’t see the difference. The more common form of preimplantation genetic diagnosis is to test for so-called aneuploidies (times where the number of chromosomes that are in the embryo are not expected to be able to lead to a normal baby). Aneuploidies are very common. We are all familiar that they occur more frequently as we get older. The most common one is Trisomy 21, which some people used to call Down Syndrome. What trisomy means is that instead of having a double dose of chromosomes (which is what you are supposed to do) you end up with a triple dose. So, any time the count of the chromosomes is improper (either three or only one), it’s abnormal; three would be a trisomy, one would be a monosomy. When people are screening embryos today, they are using very sophisticated techniques that give them a very accurate diagnosis of the chromosomal counts in the cells that have been biopsied. The shortcoming that we see in this: is that embryos are not all uniform in their distribution of chromosomes. Different cells in an embryo may have different chromosomal counts. When the biopsies are done on the embryo they may be sampling a part of the embryo that is not likely to actually represent the part that is going to become the baby. On that basis, they are throwing these embryos away.