Preimplantation genetic diagnosis (PGD) is a technique used to identify genetic defects in embryos created through in vitro fertilization (IVF) before transferring them into the uterus. Because only unaffected embryos are transferred to the uterus for implantation, PGD provides an alternative to current post-conception diagnostic procedures, i.e., amniocentesis or chorionic villus sampling, which are frequently followed by pregnancy termination if results are unfavorable.
PGD is performed in conjunction with IVF and is offered to fertile and infertile couples. PGD is recommended when embryos may be affected by a certain genetic condition. Only healthy and normal embryos are transferred into the mother's uterus, thus greatly reducing the risks of adverse outcomes such as miscarriages, pregnancy termination (after positive prenatal diagnosis) or birth defects (physical and/or mental).
The three major categories of chromosomal disorders that PGD may screen for are Aneuploidy, Translocations and Single Gene Defects.
PGD primarily has been applied to women of advanced maternal age, multiple pregnancy losses or multiple failed IVF attempts. The IVF team uses PGD to select embryos with a normal number of chromosomes in order to increase the chances of a normal pregnancy. Some parents may have never achieved a viable pregnancy without using PGD because previous conceptions resulted in aneuploid embryos or chromosomally unbalanced embryos and were spontaneously miscarried.
Chromosomes are string-like structures found in the center of the cell, the nucleus. Chromosomes contain genes that are made of DNA. Therefore, our inherited information is housed on the chromosomes. A translocation is a change in chromosome structure in which chromosomes are attached to each other or pieces of different chromosomes have been interchanged.
Individuals with balanced translocations typically have no medical issues though some do have fertility problems such as reduced fertility. The concern regarding having a balanced translocation is that, though the individual is healthy, the egg or sperm of that individual can have an unbalanced chromosome make-up that leads to the resultant embryo or pregnancy being unbalanced.
The presence of an unbalanced translocation can lead to an embryo not implanting, a pregnancy being lost or a child being born with mental and physician problems. As such, individuals with a translocation may experience multiple pregnancy losses or have a child affected with physician and mental problems that may be lethal.
Single Gene Defects
PGD can be applied to cases where couples are at risk of passing on an inherited genetic disease because of a Single Gene Disorder (SGD).
PGD for SGD can alert the couple to serious genetic conditions that they may want to avoid. In this case, PGD allows the IVF team to identify affected embryos and transfer only embryos unaffected by the gene, reducing the risk of affected children. The most common SGDs are:
- Cystic fibrosis
- fragile X
- Myotonic Dystrophy
- Tay Sachs
- Sickle Cell Disease