GENETIC SCREENING
In medicine and (clinical) genetics pre-implantation 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.
The term pre-implantation genetic screening (PGS) is used to denote procedures that do not look for a specific disease but use PGD techniques to identify embryos at risk. PGD is a poorly chosen phrase because, in medicine, to "diagnose" means to identify an illness or determine its cause. An oocyte or early-stage embryo has no symptoms of disease. They are not ill. Rather, they may have a genetic condition that could lead to disease. To "screen" means to test for anatomical, physiological, or genetic conditions in the absence of symptoms of disease. So both PGD and PGS should be referred to as types of embryo screening. Procedures performed on sperm cells before fertilization may instead be referred to as sperm sorting, although the methods and aims partly overlap with PGD.
A more recent application of PGD is to diagnose late-onset diseases and (cancer) predisposition syndromes. It can be argued that PGD for late-onset diseases is unethical since the individuals remain healthy until the onset of the disease, usually in the fourth decade of life. On the other hand, the high probability or certainty of developing some disorders, and their incurable nature, can lead to a stressful life, waiting for the first symptoms to occur and to an early death. Increasingly, PGD is also used for sex selection for non-medical reasons. A 2006 survey found that 42 per cent of clinics that offer PGD have provided it for this reason. Nearly half of these clinics perform it only for “family balancing”, which is where a couple with two or more children of one sex desire a child of the other, but half do not restrict sex selection to family balancing. In India, this practice has been used to select only male embryos although this practice is illegal. Opinions on whether sex selection for non-medical reasons is ethically acceptable differ widely, as exemplified by the fact that the ESHRE Task Force could not formulate a uniform recommendation.
