Both Preimplantation Genetic Screening (PGS) and Preimplantation Genetic Diagnosis (PGD) are types of Preimplantation Genetic Test, a technique that helps to analyse the DNA from oocytes and embryos for HLA-typing as well as to detect genetic defects and abnormalities in the embryos that have been produced using in vitro fertilization technique. The procedure is performed prior to the placement of the embryo, during IVF. Genetic testing is a vital resource for numerous couples and especially those having a family or personal history of genetic defects. Preimplantation genetic testing is an alternative for post-conception diagnostic procedures like amniocentesis or chorionic villus sampling which, if not favourable, may call for the need pregnancy termination which is a very difficult decision. The procedure gives the doctors the freedom to transfer only the unaffected embryo which naturally results in a healthy pregnancy.
Also known as aneuploidy screening, PGS refers to a set of techniques that help to test whether the embryo, obtained via IVF/ICSI, is aneuploid i.e. whether they have an abnormal number of chromosomes or if there is any structural abnormality. Women aged 35 or older are considered to be at a higher risk of having babies with certain genetic disorders irrespective of whether they have a family or personal history of the same. The eggs present inside the ovaries are immature and as a woman ovulates. These pass through a phase of cell division known as meiosis, which causes the halving of the chromosomal material such that each parent contributes 23 chromosomes to the pregnancy resulting in 46 number of chromosomes post fertilization. Ageing can alter the number of chromosomes in the eggs of the female partner, thereby leading to chromosomal abnormalities.
However, it is pertinent to note that chromosomal abnormalities are not age restricted which means that these can happen in the pregnancies of younger women as well. Hence, a risk assessment, based merely upon the intended mother’s age is not adequate.
Preimplantation genetic screening has the following benefits:
Unlike PSG, PGD is recommended in cases where either one or both of the intended parents have a known genetic abnormality so as to determine whether the embryo carries the same condition. One of the greatest benefits of PGD is that it helps to negate the need for selective abortion as the baby is most likely to be free of any genetic abnormality.
PGD can help in the diagnosis of 3 major groups of diseases – sex-linked disorders, single-gene defects and chromosomal disorders.
Sex-linked disorders- These include a wide range of sex-linked recessive and dominant disorders. The former includes conditions like haemophilia and fragile X syndrome whereas the latter includes disorders like pseudo hyperparathyroidism and vitamin D–resistant rickets.
Single-gene defects- These include conditions like cystic fibrosis, sickle cell anaemia, and Huntington disease which are diagnosed with molecular techniques using polymerase chain reaction (PCR) amplification of DNA from a single cell.
Chromosomal disorders- These include a wide gamut of chromosomal rearrangements including translocations, inversions and deletions which are detected using fluorescent in situ hybridization (FISH) technique.