Preimplantation genetic diagnosis (PGD or PIGD) (also known as embryo screening) refers to procedures that are performed on embryos prior to implantation, sometimes even on oocytes prior to fertilization. PGD is considered another way to prenatal diagnosis. Its main advantage is that it avoids selective pregnancy termination as the method makes it highly likely that the baby will be free of the disease under consideration. PGD thus is an adjunct to assisted reproductive technology, and requires in vitro fertilization (IVF) to obtain oocytes or embryos for evaluation.
Indications and applications
Currently, there are mainly two groups of patients for which PGD is being applied:
In the first group PGD is used to look for a specific disorder in couples with a high risk of transmitting an inherited condition. This can be a monogenic disorder, meaning the condition is due to a single gene only, (autosomal recessive, autosomal dominant or X-linked disorders) or a chromosomal structural aberration (such as a balanced translocation). PGD helps these couples identify embryos carrying a genetic disease or a chromosome abnormality, thus avoiding the difficult choice of abortion. In addition, there are infertile couples who carry an inherited condition and who opt for PGD as it can be easily combined with their IVF treatment.
The second group consists of couples who undergo IVF treatment and whose embryos are screened for chromosome aneuploidies. The technique is not used to obtaining a specific prenatal diagnosis but rather for screening, properly referred to as preimplantation genetic screening (PGS), to increase the chances of an ongoing pregnancy. The main applications for PGS are an advanced maternal age, a history of recurrent miscarriages or repeated unsuccessful implantation. As the results of PGS rely on the assessment of a single cell, PGS has inherent limitations as the tested cell may not be representative of the embryo and embryo mosaicism may not be be clinically significant. Further, studies have not shown that IVF success rates in terms of live births are better when PGS is used, and there is some concern that a biopsy may lower success rates. It has also been proposed for patients with obstructive and non-obstructive azoospermia.