PGD is a laboratory technique, which involves genetically analysing the embryos from an IVF cycle in order to detect genetic abnormalities and chromosomes before the transfer. This way only healthy embryo is placed in the uterus, thus avoiding abortion or having to contemplate a voluntary interruption of pregnancy.
There are different types of preimplantation genetic diagnosis:
- Aneuploidy: numerical study of alterations in the chromosomes of the embryo, ruling out Down syndrome, Klinefelter, and others.
- Translocations: changes in chromosome structure.
- Monogenic Disorders: exclusion of inherited diseases such as haemophilia A, B, thalassemia, Multiple endocrine neoplasia (MEN-I, MEN-II,) and others.
- Rare Diseases: are progressive with a poor prognosis in most cases, leading to chronic and serious illnesses.
These are complementary techniques used in In Vitro Fertilization (IVF). The embryos resulting from the fertilization of the oocytes with semen are maintained in culture and when they reach the appropriate stage of cell division, proceed to biopsy for genetic studies.
Biopsied embryo forms:
- Polar body biopsy: with this analysis, we are only able to determine information on the genetic contribution of the woman and no paternal chromosome abnormalities nor those that may arise after fertilization are detected.
- Blastomeric : are the cells that form the embryo. A blastomere is sucked through a hole in the embryo when it has an adequate development division, usually the third day after fertilization.
- Embryo: which does not affect foetal development, the process of cell division stops momentarily however the embryo will reach the same number of cells as it would have normally developed.
Two types of techniques are mainly used for this study:
- Fluorescence in situ hybridization (FISH): is the use of DNA probes (DNA fragments) specific for certain chromosomes or chromosomal regions. Each probe is labelled with a different fluorochrome and these fluorescent probes are applied to the biopsied cell, in order to bind chromosomes so as to count the number of each of the chromosomes (colour) is in each cell.
- Polymerase chain reaction (PCR): is the amplification of certain DNA sequences specifically to a detectable level for observation so that the gene can be analysed. It is primarily used for the diagnosis of monogenic diseases.