Preimplantation Genetic Diagnosis (PGD) is a procedure which is used to screen embryos during the In Vitro Fertilization (IVF) process. While PGD is not used for all IVF patients, many do opt for the procedure, because it allows them to select the healthiest embryos for transfer, increasing the chances of having a healthy baby. This procedure was first performed in 1988, and it has significantly improved since that time.

You may also hear PGD referred to as Preimplantation Genetic Screening (PGS) or just “embryo screening.” It can be performed on a single oocyte, or human egg, or it can be performed on an embryo. To perform PGD, a sample of the egg or embryo is carefully biopsied and then subjected to genetic screening. After the screening, a doctor can identify the healthiest embryos, and recommend one or more for transfer to the uterus of the patient.

There are two main reasons to perform PGD. The first is in the case of a couple which is prone to genetic diseases. For example, if a mother knows that she carries hemophilia, she may opt for PGD and choose not to implant embryos which test positive for the disease. A wide variety of genetic diseases can be screened for with the use of PGD, including Huntington's Disease, Fragile X Syndrome, and cystic fibrosis, among many others.

PGD is also performed to test for aneuploidy, an unusual number of chromosomes which could cause medical problems; the risk of aneuploidy is increased with older mothers. In the case of trisomy, someone has three chromosomes where there should be a pair, and in monosomy, a single chromosome is present, rather than a pair. Aneuploidy can cause a range of birth defects, and testing for it can allow people to eliminate embryos which may carry such birth defects.

There are a number of ethical concerns about PGD. Although PGD does not allow parents to select “designer babies,” it does set up situations where parents could choose to discard embryos which are prone to cancer, deafness, blindness, and other conditions which are not necessarily fatal, depending on how they are handled. Some bioethicists worry that PGD sets up a situation where people could come to value some forms of life over others. PGD also allows people to select for male or female children, another practice which is not without controversy, and some ethicists recommend the establishment of firm lines when it comes to using PGD in reproductive decisions, which has led to more debate.

PGD has some very definite benefits. For example, some embryos will fail to develop, due to profound genetic defects, and eliminating those embryos increases the chances of a healthy pregnancy. Parents can also use knowledge gained during PGD to inform health decisions for their children, and using PGD can reduce the need for additional IVF cycles by increasing the chance of producing a baby.

PGD is not, of course, flawless. It is possible to miss genetic defects, depending on which cell is biopsied, or to misdiagnose defects. Some medical professionals are also concerned that PGD could interfere with embryonic development, since it involves removal of a cell at a critical stage of development for the embryo.