Preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) are techniques that provide diagnostic information concerning an embryo prior to its transfer to the uterus. The vast majority of PGD and PGS procedures are performed by removing 1 or 2 cells (or blastomeres) of a 6- to 8-cell embryo on day 3 of embryo culture following IVF. These cells are rapidly analyzed, and, on day 5, the unaffected embryos are selected for embryo transfer.
PGD was first performed in 1989 in an effort to avoid the transfer of embryos that carried serious genetic disorders (e.g., cystic fibrosis). Thus, couples who undergo PGD do not have infertility but rather are at risk for passing a genetic disease to their children. A wide range of single-gene and chromosomal disorders can now be diagnosed with PGD, including autosomal recessive diseases (e.g., cystic fibrosis), X-linked recessive diseases (e.g., hemophilia, Duchenne muscular dystrophy), autosomal dominant diseases (e.g., Huntington’s disease), and chromosomal rearrangements (e.g., balanced translocations).
PGS is similar to PGD, but refers to screening of embryos produced in the course of fertility treatments. Thus, couples who undergo PGS include infertile patients without an underlying genetic problem. PGS is performed in an attempt to identify those embryos that are genetically abnormal so that improved embryo selection will—ideally—result in improved pregnancy rates and lower miscarriage rates.