PGD is a step added to an IVF cycle, which enables us to choose which embryos will be implanted into the mother.
2) Embryo biopsy
Preimplantation genetic diagnosis involves embryo biopsy; therefore, sensitive, accurate and safe technical methods are required.
Embryo biopsy can be performed at the following stages:
- At the polar body stage (first day after oocyte collection)
The polar body (extra genetic material) extruded from the unfertilized egg can be biopsied to screen for maternal or egg-only chromosomal disorders. Polar body biopsies do not detect problems that occur after fertilization with the sperm.
- At the 6-8-cell embryo stage (three days after oocyte collection)
Embryo biopsy may be performed after 3 days of culture in the laboratory. The embryos are typically 8-cell embryos on Day-3 and the process involves the removal of one to two cells (blastomeres). After the biopsy embryo(s) of the best quality that are not affected by the genetic disorder or chromosomal abnormality are selected for transfer to the uterus. For day 3 embryo biopsies, the embryo is usually transferred “fresh” following two additional days of culture in the laboratory (Day-5 embryo transfer).
- At the blastocyst stage (five to six days after oocyte collection)
3) Genetic evaluation
The cell(s) obtained from the biopsy are immediately submitted to genetic evaluation. This can be accomplished in several ways depending on what is being studied. For example, in single gene disorders such as cystic fibrosis, PCR analysis is used to detect a specific mutation in the DNA of the embryo. If on the other hand the goal is to look for defects involving entire chromosomes FISH technology using florescent signals is employed to detect extra, missing, or abnormally structured chromosomes in the embryo.
Most recently, exciting advancements have been made to develop techniques that are able to check the entire genome (all genes within the chromosomes) of the cell(s) tested and screen for both known and unknown genetic defects that may be present in the embryo. This technology is called Comparative Genomic Hybridization (CGH) and employs computer software and “DNA chips” to check for aneuploidy, translocations and single gene disorders in one single test.