Preimplantation Genetic Diagnosis

Preimplantation genetic diagnosis or PGD

Preimplantation genetic diagnosis or PGD is a process in which embryos developed outside the womb are tested for particular genetic characteristics, usually genetic abnormalities that cause serious disease, before being transferred to a woman’s uterus.  PGD owes its existence to advances in the world of reproductive medicine and genetics that occurred in the late 20th century.  As understanding of the genetic basis of inherited disorders increased, so did the number of tests available to detect specific disorders.  The use of these tests in prenatal diagnosis allowed the detection of genetic abnormalities in a human fetus in utero.  As genetic medicine progressed, so did work on IVF.

Scientists developed methods to perform genetic tests on a small amount of genetic material taken from an egg or embryo.  This new technique, PGD, permits doctors and prospective parents to select embryos for implantation that do not have a genetic abnormality associated with a specific disease, such as  cystic fibrosis, or, alternatively, to select embryos that possess a genetic trait deemed desirable, such as a tissue type that matches that of an ailing sibling. 

There are alternatives to PGD.  Prospective parents at risk of passing a genetic condition to their offspring can choose to avoid pregnancy, conceive using donor egg or sperm from an individual who does not carry the mutation in question, proceed with a pregnancy but undergo a prenatal diagnostic test (and possibly terminate the pregnancy if it reveals a gene mutation) or accept the possibility that their child could be born with a genetic abnormality. 

In the ten years since PGD was first made available to facilitate embryo selection, more than 1,000 babies have been born worldwide following a preimplantation genetic test.  Inherited chromosome abnormalities and single gene disorders including cystic fibrosis, Tay Sachs disease, muscular dystrophy and sickle cell anemia have been detected with PGD.  While originally used by families with a known genetic disease who were not infertile, more recently PGD has been used as an adjunct to standard IVF to detect chromosomal abnormalities, called aneuploidy, arising during egg or embryo development. Some providers recommend PGD for patients over 35 or those with repeated IVF failure.  Given that many IVF patients are over 35, aneuploidy screening may soon account for the majority of PGD procedures.