Preimplantation genetic diagnosis (PGD), also called Preimplantation Genetic Testing (PGT), is a procedure used prior to implantation to help identify genetic defects within embryos created through in vitro fertilization to prevent certain diseases or disorders from being passed on to the child. In most cases, the female, male, or both partners have been genetically screened and identified to be carriers of potential problems.
How is the PGD performed?
The preimplantation genetic diagnosis begins with the normal process of in vitro fertilization that includes: ovary stimulation through medication, egg retrieval, and fertilization in a laboratory. Over the next three days the embryo will divide into 8 cells. The preimplantation genetic diagnosis involves 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 known as polymerase chain reaction (PCR).
3. Finally, by molecular analysis, the DNA sequence code is evaluated to determine if the inheritance of a problematic gene is present.
Once the PGD procedure has been performed and embryos free of genetic problems have been identified, implantation will be attempted through embryo transfer, intracytoplasmic sperm injection (ICSI), or zygote intrafallopian transfer (ZIFT).
Who can benefit from PGD?
Preimplantation genetic diagnosis can benefit any couple at risk for passing on a genetic disease or condition. The following is a list of the type of individuals 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 associated with chromosomal concerns
PGD has also been used for the purpose of gender selection. However, discarding embryos based only on gender considerations is an ethical concern for many people.
What does PGD look for?
Preimplantation genetic diagnosis looks for genetic and chromosomal problems that place the couple at risk for birth defects or spontaneous miscarriage. Research shows that PGD identifies the presence of the following disorders and the list continues to grow as technology improves:
- Recessive sex-linked disorders such as hemophilia, fragile X syndrome, and most neuromuscular dystrophies
- Dominant sex-linked disorders such as Rett syndrome, incontinentia pigmenti, pseudohypererp arathydroidism, and vitamin D-resistant rickets
- Single gene disorders such as cystic fibrosis, Tay-sachs, Huntington disease, and sickle cell anemia
- Chromosomal rearrangements such as 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 implantation thus reducing the need for amniocentesis later in pregnancy.
- The procedure is performed before implantation thus allowing the couple to decide if they wish to continue with the pregnancy.
- The procedure enables couples to pursue biological children who might not have done so otherwise.
- The procedure may help reduce the costs normally associated with birth defects.
What are the concerns of PGD?
The following are considered concerns or disadvantages associated with the use of PGD:
- Many people believe that because life begins at conception and that the destruction of an embryo is the destruction of a person. In practice, the PGD procedure usually results in a small number of discarded embryos.
- In some cases, a genetically defective fertilized egg will mature without the presence of disorder or disease. The probability of disorder development should be a topic of discussion with the healthcare provider.
- While PGD helps reduce the chance of conceiving a child with a genetic factor, it can not completely eliminate this risk. In some cases, further testing done during pregnancy is needed to ascertain if a genetic factor is still possible.
- Although genetically present, some resulting diseases only 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