PGD/S involves genetic procedures performed on embryos prior to implantation. PGD/S works in conjunction with assisted reproductive technology and requires in vitro fertilisation (IVF).
PGD is the diagnosis of specific genetic and chromosome abnormalities in couples who are at high risk of transmitting these abnormalities to their children. PGS is the aneuploidy screening of as many chromosomes as possible in embryos from patients with subfertility with the aim of increasing their chances of a normal pregnancy.
Embryo biopsy remains the main approach to removing DNA for genetic analysis. Cells must be removed from the embryo and must contain a nucleus to be suitable for genetic analysis. Scientific studies thus far have shown that further development of the embryo is not impaired by the biopsy.
The techniques used for PGD/S are fluorescence in situ hybridization (FISH), polymerase chain reaction (PCR), array – Comparative Genomic Hybridisation (array-CGH) and karyomapping. FISH uses fluorescently tagged DNA probes which bind to their complementary sequence on a specific chromosome and may be visualised under a fluorescence microscope. PCR is a technique that amplifies a piece of DNA into thousands of copies of a specific DNA sequence. Single cell PCR is a technically demanding procedure. Array-CGH is a genome-wide screening tool for the detection of copy number imbalances. Karyomapping determines whether embryos have inherited gene defects by analysing DNA from the embryos and close relatives.
The success of the complete PGD/S procedure is dependent on various factors: fertilization, embryo quality, successful biopsy, genetic testing procedure and result interpretation. This whole process influences the number of healthy embryos available for transfer.