PGD and PGS: Two Methods of Gaining Information Prior to Implantation
Preimplantation genetic diagnosis (PGD) is a technique used to identify many inherited diseases. PGD uses DNA amplification to identify embryos with specific mutations of single genes, which may have been acquired from the mother, or father or both. What PGD can do: 1. PGD can diagnose embryos at risk for some specific genetic diseases if the parent(s) are known to be carriers, and the molecular genetic basis of that disease is known. 2. PGD identification enables elimination of those embryos carrying the genetic mutation that causes the disease in question. It cannot repair those mutations. What PGD cannot do: 1. PGD cannot guarantee that the baby will be free of all diseases or birth defects because the genetic basis for many defects is unknown. At this time, it is impractical or impossible to screen for most diseases, such as diabetes and cancer, or birth defects such as cleft lip and palate. 2. PGD cannot diagnose all diseases, even if the genetic basis is known, because some of the rarer diseases do not yet have available DNA probes. 3. PGD cannot determine traits, such as eye color, height, intellectual or athletic abilities. 4. PGD is not perfect, despite how sophisticated it is. Errors in diagnosis can occur, albeit at a very low rate. Confirmation of the correct diagnosis should be done by chorionic villus sampling (CVS) or amniocentesis, once the pregnancy is established. The second type of genetic analysis is what we like to call Preimplantation Genetic Screening (PGS) to look for abnormalities in entire chromosomes missing or extra chromosomes or multiple complex abnormalities in chromosome numbers. What PGS can do: 1. PGS can screen for abnormalities in 9 out of the 23 chromosome pairs. Currently it is not technically possible to screen for abnormalities in the other 14 chromosome pairs. 2. PGS can help to reduce the risks of miscarriage, commonly due to Monosomy X (one X chromosome) or Trisomy 16 (three of chromosome 16). 3. PGS can help to significantly decrease the risk of Down Syndrome (Trisomy 21) and Trisomy 18, as well as abnormalities in numbers of sex chromosomes (X and Y) (These are among the few abnormalities in fetuses that can survive to the time of amniocentesis and birth). 4. PGS can reduce the number of embryos one must transfer to find the embryos most likely to succeed. 5. PGS may help couples experiencing multiple IVF failures to determine if the failed implantations may be due to aneuploidy (chromosomal abnormalities). 6. PGS can determine the gender of the embryo. What PGS cannot do: 1. PGS cannot screen for specific genetic diseases couples at risk need PGD. 2. PGS cannot guarantee that the baby will be free of all diseases or birth defects. 3. PGS is not perfect. The detection rate is between 90-93% for the chromosomes analyzed, which is why we still recommend CVS or amnio as a confirmation of PGS findings.