Preimplantation Genetic Testing (or PGT) is a test which is performed after IVF is completed, when the oocytes have been fertilized and the embryos start dividing. A genetic test can be carried out once the embryo has reached the 8-cell stage (around the 3rd day of development), by removing a single cell (blastomere biopsy). Then, the embryo is returned to the incubator for further growth. However, it is recommended to perform the biopsy once embryos have become blastocysts (day 5-6). By the 5th day of development, the embryos will have divided into approximately 100 cells and have formed the outer layer that will eventually become the placenta. At this stage, a trophoblast biopsy can be performed, by removing a few cells from the outer layer. This procedure does not affect the embryo’s quality or ability to implant.