Preimplantation genetic testing (PGT) is a method that facilitates the selection of genetically normal embryos to be transferred into the mother’s uterus. It implies a previous treatment by IVF/ICSI. Preimplantation diagnosis is a very early form of prenatal diagnosis that allows the elimination of embryos bearing genetic abnormalities.
Preimplantation genetic testing (PGT) is based on the study of DNA of human embryos to select those that meet certain characteristics and/or to eliminate those that carry some type of congenital defect. DNA samples are obtained by embryo biopsy whereby some cells are extracted for analysis. The biopsy is usually performed on day 5 after oocyte retrieval (blastocyst stage) because the most informative results can be obtained and the manipulation is the least harmful at this stage. Basically, the technique consists of separating 2-4 cells of the trophectoderm (the part of the blastocyst that will subsequently be incorporated into the future placenta) without touching the inner cell mass (embryoblast, the part of the blastocyst giving rise to the body of the future fetus). The DNA samples isolated from the extracted cells are analyzed by fluorescent in situ hybridization (FISH) or by the polymerase chain reaction (PCR).
Within the concept of preimplantation genetic diagnosis, two different types can be distinguished, both available at the MAR&Gen Clinic:
• PGD: preimplantational genetic diagnosis. It is the diagnosis of the embryo's genotype regarding the presence or absence of the allele that causes a disease or the chromosomal alteration carried by the parents.
• PGS: preimplantational genetic screening. It is the selection of chromosomally normal embryos from a cohort in which the proportion of chromosomally abnormal embryos is suspected to be elevated above normal. It is also called preimplantation testing for aneuploidies (PGTa)
PGD is a very reliable technique since the abnormal allele is inherited and therefore present in all cells of the embryo. Consequently, the information obtained from only 2-3 cells reflects the condition of the entire embryo. On the other hand, PGS is less reliable since chromosomal abnormalities appear "de novo" in preimplantation embryos and, in most cases, they only affect some cells of the embryo while others remain healthy, a situation known as genetic mosaicism. The results obtained with a few cells are therefore not necessarily representative of the state of the entire embryo. Furthermore, it is known that some mosaic embryos can correct the existing anomaly even after implantation in the uterus. It is therefore important to provide each couple with exhaustive information on the status of their embryos in order to reach a correct decision as to whether or not to transfer mosaic embryos.