Preimplantation genetic diagnosis (PGD), also called Preimplantation GeneticTesting (PGT), is a procedure used prior to implantation to help identifygenetic defects within embryos created through in vitrofertilization to prevent certain diseases or disorders from being passedon to the child. In most cases, the female, male, or both partners havebeen genetically screened and identified to be carriers of potential problems.
How is the PGD performed?
The preimplantation genetic diagnosis begins with the normal process ofin vitro fertilization that includes: ovary stimulation through medication,egg retrieval, and fertilization in a laboratory. Over the next three daysthe embryo will divide into 8 cells. The preimplantation genetic diagnosisinvolves the following steps:
1. First, a one or two cells are removed from the embryo.
2. Next, DNA is retrieved from the cell and copied through a process knownas polymerase chain reaction (PCR).
3. Finally, by molecular analysis, the DNA sequence code is evaluatedto determine if the inheritance of a problematic gene is present.
Once the PGD procedure has been performed and embryos free of geneticproblems have been identified, implantation will be attempted throughembryo transfer, intracytoplasmicsperm injection (ICSI), or zygote intrafallopiantransfer (ZIFT).
Who can benefit from PGD?
Preimplantation genetic diagnosis can benefit any couple at risk for passingon a genetic disease or condition. The following is a list of the type ofindividuals who are possible candidates for PGD:
- Women ages 35 and over
- Carriers of sex-linked genetic disorders
- Carriers of single gene defects
- Those with chromosomal disorders
- Women experiencing recurring pregnancy loss associatedwith chromosomal concerns
PGD has also been used for the purpose of gender selection. However, discardingembryos based only on gender considerations is an ethical concern for manypeople.
What does PGD look for?
Preimplantation genetic diagnosis looks for genetic and chromosomal problemsthat place the couple at risk for birth defects or spontaneous miscarriage.Research shows that PGD identifies the presence of the following disordersand the list continues to grow as technology improves:
- Recessive sex-linked disorderssuch as hemophilia, fragile X syndrome, and most neuromuscular dystrophies
- Dominant sex-linked disorderssuch as Rett syndrome, incontinentia pigmenti, pseudohypererparathydroidism, and vitamin D-resistant rickets
- Single gene disorders such ascystic fibrosis, Tay-sachs, Huntington disease, and sickle cell anemia
- Chromosomal rearrangements suchas translocation, inversion, deletions and Aneuploidy
What are the benefits of PGD?
The following are considered benefits or advantages of PGD:
- The procedure is performed before implantationthus reducing the need for amniocentesis later in pregnancy.
- The procedure is performed before implantationthus allowing the couple to decide if they wish to continue with the pregnancy.
- The procedure enables couples to pursue biologicalchildren who might not have done so otherwise.
- The procedure may help reduce the costs normallyassociated with birth defects.
What are the concerns of PGD?
The following are considered concerns or disadvantages associated withthe use of PGD:
- Many people believe that because life begins atconception and that the destruction of an embryo is the destruction ofa person. In practice, the PGD procedure usually results in a small numberof discarded embryos.
- In some cases, a genetically defective fertilizedegg will mature without the presence of disorder or disease. The probabilityof disorder development should be a topic of discussion with the healthcareprovider.
- While PGD helps reduce the chance of conceiving a child with a geneticfactor, it can not completely eliminate this risk. In some cases, furthertesting done during pregnancy is needed to ascertain if a genetic factoris still possible.
- Although genetically present, some resulting diseasesonly generate symptoms when carriers reach middle age.
Compiled using information from the following source:
Infertility: A Comprehensive Text-Second Ed. Seibel,Machelle M., Ch. 42.
American Society for Reproductive Medicine (ASRM), http://www.asrm.org