Preimplantation Genetic Diagnosis (PGD) The advent of preimplantation genetic diagnosis (PGD) makes it possible for couples that have a genetically transferable disease to virtually eliminate the possibility of passing it to their child. PGD is performed on embryos either at the six to eight cell stage or at the blastocyst stage. At the 6-8 cell stage, one cell is removed for testing. At the blastocyst stage 3-4 cells are removed from the trophoblast. The removal of the cell(s) does not harm the developing embryo. The cell(s) that have been removed from the embryo are tested for the genetic disease. Only embryos that do not carry the disease are transferred to the uterus.