eMedicine - Preimplantation Genetic Diagnosis : Article by JJ Marik, MD: "Definition
PGD combines recent advances in genetics and reproductive medicine. For PGD, the early embryo is examined after IVF for inherited diseases or to determine the sex of the embryo for sex-related genetic disorders. The process starts with a basic IVF. When the embryo is at the 6- to 8-cell stage, 1-2 cells (blastomeres) are removed and sent to the genetic laboratory for diagnosis using either polymerase chain reaction (PCR) or fluorescence in situ hybridization (FISH) techniques, depending on which disease is being sought. The unaffected embryos are then transferred into the mother's uterus.
In certain situations in which the genetic problem is only with the female, a polar body can be removed from the eggs and tested before fertilization."