Pre-implantation Genetic Diagnosis
Preimplantation genetic diagnosis (PGD) is a reproductive technology used to identify genetic defects in embryos created in an IVF cycle. The technique is used when one or both parents have a known genetic abnormality. The embryos are tested for a panel of chromosome abnormalities so that only unaffected embryos are used to create a pregnancy.
PGD was developed as an alternative to prenatal diagnosis and potential termination of a fetus affected by genetic disease.
The process of genetic testing with PGD is done with IVF. As with all IVF procedures, eggs are removed after ovarian stimulation and fertilized in a laboratory. Approxinately three days after fertilization, the embryo will have developed around eight cells. One to two cells are removed by biopsy and are tested for genetic abnormalities.
A further technique called comparative genomic hybridization (CGH) allows doctors to remove cells from the embryos at a later stage. At five days, the embryo has about 100 cells, and removing cells at this stage can cause less damage. This provides greater certainty about whether or not genetic abnormalities in the embryo exist.
Candidates for PGD include those with:
- A history of recurrent miscarriage due to genetic disorders
- Repeated IVF failure
- Male partners with severe male factor infertility
- An exisiting child with a genetic disease
- Requiring a tissue match for an ill sibling
- A family history of X-linked disorders
- Chromosome translocations, which can cause implantation failure, recurrent pregnancy loss, mental or physical problems in children
- Autosomal recessive diseases, such as cystic fibrosis, sickle cell anemia and Tay Sachs disease
- Autosomal dominant diseases, including dwarfism and Huntington's disease
PGD may not be possible for women over 35 and women with high FSH levels. Pregnancy rates are lower for this group and enough embryos may not be produced to undergo testing. The male is also required to have a normal sperm count for adequate testing to be done.