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Pre-implantation Genetic Diagnosis

Pre–implantation genetic diagnosis (PGD)


Preimplantation genetic testing is a technique used to identify genetic defects in embryos created through in vitro fertilization (IVF) before pregnancy. Preimplantation genetic diagnosis (PGD) refers specifically to when one or both genetic parents has a known genetic abnormality and testing is performed on an embryo to determine if it also carries a genetic abnormality. In contrast, preimplantation genetic screening (PGS) refers to techniques where embryos from presumed chromosomally normal genetic parents are screened for aneuploidy.

Because only unaffected embryos are transferred to the uterus for implantation, preimplantation genetic testing provides an alternative to current postconception diagnostic procedures (ie, amniocentesis or chorionic villus sampling), which are frequently followed by the difficult decision of pregnancy termination if results are unfavorable. PGD and PGS are presently the only options available for avoiding a high risk of having a child affected with a genetic disease prior to implantation.

Two main techniques are used for the genetic assessment:

  • Polymerase chain reaction (PCR) In PCR, multiple copies of the gene of interest are made by a process of amplification. This amplification process allows the identification of very small amounts of DNA to make the diagnosis.
  • Fluorescent in situ hybridization (FISH) FISH allows the laboratory to count the number of chromosomes in an isolated cell. This technique is used primarily for expected abnormalities in chromosome number, such as Down syndrome, or translocations (defects in the structure of the chromosome).