The evolution made in various fields of assisted reproductive technology has contributed in many ways in the achievement of a pregnancy and the delivery of a healthy baby.
The term PGT (Preimplantation Genetic Testing) is used internationally to describe genetic testing, of embryos in the pre-implantation stage. It is therefore a technique that allows the investigation of the embryo’s genetic background before implantation. Therefore, it is obvious that it can only be performed in cases of IVF treatments.
If PGT IVF treatment is chosen, then the sequence of the steps is as usual: controlled ovarian stimulation is performed and the woman is subjected to oocyte pick up. The retrieved mature eggs get fertilized by ICSI (Intra Cytoplasmic Sperm Injection) and the embryos are cultured to the blastocyst stage (day 5 of the culture). Embryos that reach blastocyst stage are subjected to trophectoderm biopsy and then the embryos get frozen by the procedure of vitrification. Freezing of the embryos is performed to provide the essential time that is required for the genetic test results.
The genetic testing of the embryos is implemented by a small number of embryo cells that are removed during the biopsy. While the embryos are frozen, the removed cells are sent to the genetic laboratory to be analyzed. The difference in this kind of IVF treatment is that after the genetic lab testing, only the healthy embryos are chosen to be transferred, in order to achieve a pregnancy.
All couples that choose to perform preimplantation genetic testing are also offered embryo monitoring through the MIRI Time Lapse incubator. This particular incubator provides monitoring of the embryos via an internal camera throughout the whole incubation period. In this way, embryos can be assessed by the monitor imaging, avoiding thus the disturbance of their optimal culture conditions. MIRI Time Lapse incubator allows the embryologist to evaluate the embryo progress throughout their culture, from the time of the fertilization to the time of their biopsy. Therefore, the time of the biopsy is chosen for each embryo appropriately, depending on its developmental stage.
Until recently, Preimplantation Genetic Testing was known under different terms, which represented a particular case. The term PGS (Preimplantation Genetic Screening) was used when preimplantation testing involved the analysis of the number and the structure of chromosomes. In other words, the embryo in this case is screened to determine whether it is euploid (normal number of chromosomes) or aneuploid (abnormal number of chromosomes). Preimplantation Genetic Diagnosis (PGD) aims to analyze the genetic background of the embryo regarding a particular gene- associated with a monogenetic disease. Therefore, the diagnosis is not related to the chromosomal composition of the embryo, but aims to assess whether or not the embryo suffers from a particular genetic disease (eg thalassemia, cystic fibrosis, etc.).
The current international terminology of preimplantation genetic testing has been modified to accurately describe the specific type of the embryo genetic analysis. The technique is characterized generally as PGT (Preimplantation Genetic Testing) and the following terminologies are used where appropriate:
- PGT-A: PGT for Aneuploidy – formerly known as PGS
- PGT-M: PGT for diagnosis of monogenic diseases (PGT for Monogenic Diseases) – formerly known as PGD
- PGT-SR: PGT for the analysis of structural chromosomal rearrangements (PGT for Structural Rearrangements)
The main idea behind preimplantation genetic testing (PGT- A/ PGS) is based on the fact that a euploid embryo has a better chance of implantation and clinical pregnancy comparing with an aneuploid embryo, which is more likely not to implant or to lead to a miscarriage. In addition, preimplantation genetic testing prevents transfer of embryos, which although fully viable, may carry syndromes due to aneuploidies (eg Down syndrome, Kleinefelter, Turner, etc.). Preimplantation genetic testing for aneuploidies (PGT-A) can improve the IVF results, as it increases the probability of implantation and reduces miscarriage rates. On the other hand, preimplantation genetic diagnosis (PGT-M/ PGD) aims to genetically analyse the embryos for a specific genetic disease, for which the couple usually has a relevant family history.
The choice of PGT- IVF treatment is made after consultation of the couple with the physician and geneticist, based primarily on the patient’s reproductive age, gynecological history and the couple’s infertility history. Analyzing the potential risks and benefits of each case, all the innovative methods in Assisting Nature are combined to turn your dreams into reality.