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The term PGT (Preimplantation Genetic Testing) is used internationally to describe genetic testing, i.e. testing the DNA of embryos, at the pre-implantation stage. In other words, it is a technique that evaluates the genetic status of the embryo, before it implants. Obviously, pre-implantation genetic testing can only be done if the couple has undergone IVF treatment. The embryos resulting from the IVF cycle are screened and only healthy embryos are selected for transfer to the woman’s uterus to achieve implantation and pregnancy.

Genetic testing of embryos is done by removing a small number of their cells, which are tested in the genetic laboratory. The procedure by which the cells of the embryo are removed is called a biopsy.

Until recently, preimplantation genetic testing was known by two other names, each representing a specific case. The term PGS (Preimplantation Genetic Screening) is the case in which an embryo is tested for the number and morphology of the chromosomes in its cells. In other words, the embryo is tested to diagnose whether it is euploid (normal number of chromosomes) or aneuploid (abnormal number of chromosomes).

Preimplantation Genetic Diagnosis (PGD- Preimplantation Genetic Diagnosis) aimed to test the embryo for the existence of a gene associated with a single-gene disease. The diagnosis does not aim to test whether the embryo has a normal number of chromosomes, but its purpose is to evaluate whether or not it suffers from a genetic disease (e.g. thalassemia, cystic fibrosis, etc.).

The current international terminology of preimplantation genetic diagnosis has been modified to describe precisely the type of genetic testing that the embryos will undergo. Thus, today the following terminologies are used internationally:

PGT-A: PGT for aneuploidy – formerly known as PGS

PGT-M: PGT for monogenic diseases – previously known as PGD

PGT-SR: PGT for screening for structural chromosomal rearrangements

PGS or PGD???

The basic idea behind preimplantation genetic screening (PGS) is that a euploid embryo has a better chance of implantation and clinical pregnancy than an aneuploid embryo. Additionally, an aneuploid embryo besides implantation failure, may lead to a miscarriage or to the birth of a person suffering from syndromes.

On the other hand, preimplantation genetic diagnosis aims to test the embryo for a genetic disease, for which the couple probably has a history.

Genetic Testing Techniques

The techniques with which genetic testing is done today is usually whole genome sequencing through NGS (Next Generation Sequencing) technology. This technology allows the analysis of 24 chromosomes in a short time and at low cost. In addition, it is a cost-effective method as it has a higher sensitivity than other techniques and allows the diagnosis of abnormalities at a high level, as well as the diagnosis of cases of mosaicism.

An alternative method of diagnosis is the aCGH (array-based Comparative Genomic Hybridization) technology, which however, is now considered obsolete compared to NGS. This test can detect deviations from the normal number of chromosomes, as well as quantitative chromosomal changes of smaller extent, such as microdeletions and microduplications. In this latter case, there is a lack or excess not of the entire chromosome, but of some chromosomal part. However, apart from its advantages, this technique also shows some limitations. The limitations are related with the inability to detect balanced translocations (“exchange” of chromosome segments between two chromosomes) and inversions (body chromosome segments in the correct position, but with opposite orientation), the inability to detect point mutations in the genome or polyploidy, as and cases of mosaicism in a small percentage (less than 10-15%).