Q: What is preimplantation genetic diagnosis (PGD)?

A: PGD is a procedure for testing the genetic makeup of a very early embryo created through in vitro fertilization (IVF), before it is implanted into the womb of the woman who will carry it to term. At this stage, the embryo has about eight cells, which are typically identical; they have started to multiply but have not yet begun to differentiate. One cell is extracted and analyzed; the remaining cells are capable of developing into a complete fetus.

Most IVF protocols involve hormonal treatments that hyper-stimulate women's ovaries to produce multiple eggs, which typically generate several embryos. PGD offers a tool for selecting which embryo(s) to implant in a woman's uterus.

Q: What is the purpose of PGD?

A: PGD was developed to allow couples at risk of passing on serious genetic diseases to have children not affected by it. Since its introduction in 1990, it has been most widely used to prevent the birth of children with conditions such as Down's syndrome, Tay-Sachs disease, cystic fibrosis, sickle cell, Huntington's chorea, and Cooley's anemia.

The use of PGD increased dramatically in the early 2000s, and it is increasingly being used for other reasons. These include social sex selection, creating "savior siblings" who can provide bone marrow or other transplant tissues to sick older siblings, and selecting against embryos with genes correlated with late-onset and non-fatal conditions. Some clinics have even offered the technique for purely cosmetic traits including eye color, hair color, and skin complexion.

Q: Is PGD safe and accurate?

A: The procedure does not appear to affect embryos' or fetuses' subsequent development, though more follow-up studies of children born after PGD are needed. PGD requires the use of IVF, which involves some health risks.

Occasional false results have been reported, which are mostly attributed to "embryo mosaicism," in which the embryo's cells are not identical. These and other errors can lead to the implantation of an embryo with the undesired trait. Many clinics therefore recommend follow-up amniocentesis.

Q: How many clinics offer PGD?

A: Nearly three-quarters of US fertility clinics surveyed in 2006 by the Genetics and Public Policy Center offer PGD, including all of the larger ones. The report estimates that 4-6% of all IVF cycles include PGD. Some clinics perform the tests in-house, while others use outside services.

Q: Are there ethical concerns about PGD?

A: Using PGD to prevent the birth of children with serious heritable impairments is widely accepted in the United States, but in need of greater public consideration. Concerns about PGD have been raised by disability rights activists, who argue that this practice fosters attitudes that promote discrimination against the sick or disabled because they were not "selected against" at birth.

There is also concern that PGD is already being used to prevent implantation of embryos with less serious genetic anomalies, and that this trend will accelerate as more is learned about the human genome. The difficulty in drawing the line with this technology cannot be ignored, particularly given the inadequacy of regulation and oversight of the fertility field.

Q: Is PGD a form of eugenics?

A: PGD can be considered a crude form of eugenics, since it selects among embryos based on traits that it is believed the resulting child will develop. But the vast majority of traits, including physical ones, are influenced by multiple genes, and by environmental and social factors. PGD is currently useful mostly for single-gene or chromosomal conditions.

Q: Is PGD used for sex selection?

A: PGD can be used to select for sex. This is usually to avoid passing on serious sex-linked diseases, such as hemophilia and Duchenne's muscular dystrophy. Its use for sex selection more generally is extremely controversial.

Q: Is PGD used to detect predispositions to disease, or diseases that only occur late in life?

A: In a few instances, it is possible to test with reasonable reliability for a late-onset illness, such as Huntington's disease. PGD is now also being offered for testing predisposition to early-onset Alzheimer's, polycystic kidney disease, and several kinds of cancer to people with family histories of these conditions.

In most cases, genetic tests only provide a statistical analysis of the likelihood of getting the disease in question. For example, a woman with a particular mutation may have a 5% greater likelihood of developing breast cancer.

Q: What about using PGD to rescue another child?

A: PGD has been used in a few cases to select a "sibling-saving" embryo, or one with tissues compatible to those of an already existing child suffering from a fatal disorder. For example, a child born after a PGD procedure provided cord blood for a bone marrow transplant to a sibling with Fanconi anemia, a recessive inherited disorder that causes bone marrow failure.

This use of PGD is controversial. Some argue that it is immoral to use a child as a means to save another. Others worry about the effects of either failure or success on the "savior" child, the sick child, and family dynamics. There is also concern that parents will be tempted to approve more invasive procedures to obtain transplant tissues from their savior child if they might help the sick child.

Q: Is PGD related to Inheritable Genetic Modification (IGM)?

A: Any selection done through PGD at present involves selecting from among naturally evolved traits. IGM could theoretically introduce wholly novel traits into the human gene pool. If IGM ever became practical, it is likely that PGD would be one of the "dual-use" technologies that contributed to it. More immediately, PGD could contribute to discrimination based on disability or sex. Many argue that its use should be limited to an agreed-upon list of serious gene-related diseases such as Tay-Sachs.

Last modified March 1, 2010