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Pre-implantation Genetic Diagnosis (PGD)
 
 

Overview
Current Issues in PGD
Safety and Efficacy
Access to PGD
Concerns About Use

Overview

Pre-implantation genetic diagnosis (PGD) combines advances in genetics with assisted reproductive technology (ART). PGD is performed following a round of in vitro fertilization (IVF), in which sperm and egg are joined in a laboratory dish. With IVF, several embryos are created at a time. Using PGD, the embryos can be screened before they are transferred to the uterus.

During PGD, a single cell is removed from the embryo and tested. This cell biopsy is typically performed three to four days after fertilization. Once the cell has been extracted, its genetic material can be analyzed. Based on the results of genetic tests, parents are able to choose which embryos to implant. The procedure is typically used to a) screen out embryos carrying a genetic disease, b) predict the risk of disease, and c) improve birth rates of healthy babies following ART. In the ten years since PGD was first made available to facilitate embryo selection, more than 1,000 babies have been born worldwide following a pre-implantation genetic test.

The use of PGD to perform a chromosomal analysis is increasingly common in IVF procedures, largely to increase the chances of a successful pregnancy. PGD is frequently recommended for patients over 35 years of age (the risk of chromosomal abnormalities is higher in older women) or for patients who have experienced repeated IVF failure. Other possible but controversial applications of PGD include its use to select an embryo that is an immunological match for a sick sibling, to select the sex of an embryo purely for gender preference—that is, in the absence of a sex-linked disease risk—and to test embryos for gene mutations associated with adult onset diseases such as Alzheimer disease or mutations that indicate a heightened but uncertain risk of developing a particular disease, such as hereditary breast cancer.

In theory, any of the hundreds of genetic tests now available and the many more in development could be used to test an embryo using PGD. In addition, in the future, there will be tests for predisposition to many mild diseases, and potentially someday parents will be able to test for "traits" such as height or intelligence.

Professional guidelines for the practice of PGD are limited. The American Society for Reproductive Medicine (ASRM) issued a practice committee opinion stating that PGD appears to be a viable alternative to post-conception diagnosis and pregnancy termination, acknowledging the possibility of diagnostic errors and unknown long-term consequences on the fetus. ASRM also has issued an ethics committee opinion cautioning against the use of PGD for sex selection in the absence of a serious sex-linked disease. However, there are ASRM members who offer and advertise sex selection services for the purpose of "family balancing."

Two other organizations that have developed guidelines for PGD are the PGD International Society (PGDIS) and the European Society for Human Reproduction and Embryology (ESHRE). PGDIS has developed "Guidelines for Good Practice In PGD" that outline topics such as setting up a PGD program, patient management, IVF and PGD protocols, diagnostic techniques, embryo transfer, spare embryos, follow-up of pregnancy, and quality control and assurance – to be published in 2004. ESHRE tracks PGD outcomes on a voluntary basis, but captures primarily European data. In May 2004, ESHRE’s PGD consortium released "Best Practice Guidelines" for PGD testing. ESHRE’s guidelines are currently under review by ESHRE membership.

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Current Issues in PGD

Safety and Efficacy
Many questions remain unanswered regarding the safety of PGD for both women and the resulting children. Inherent in any IVF procedure are risks associated with the use of hormones to stimulate ovulation and the potential for an ectopic pregnancy. Often, to improve the success of IVF, more than one embryo is implanted in the uterus, increasing the likelihood of a multiple pregnancy (twins, triplets etc.) Women who carry multiple fetuses are more likely to experience pregnancy complications which could adversely affect their health, and the health of their fetuses. Additionally, little is known of the effects the removal of a cell may have on the embryo or the development of the child. Currently, approximately 20% of PGD procedures results in a pregnancy.

As stated previously, during the PGD procedure only one or two cells are removed for diagnosis. This presents challenges in the ability to correctly distinguish embryos with genetic abnormalities from those without. Therefore, it is often recommended that the results should be confirmed with a pre-natal diagnosis test such as amniocentesis or chorionic villus sampling.

Access to PGD
Not unlike any new biotechnology in a relatively nascent stage, PGD is expensive. One round of IVF typically costs around $9,000. PGD adds another $4,000 to $7,500 to the cost of each IVF attempt. A standard round of IVF results in a successful pregnancy only 10-35% of the time (depending on the age and health of the woman), and a woman may need to undergo subsequent attempts to achieve a viable pregnancy. As a result, a successful pregnancy is very costly – and cost-prohibitive – for most women.

Although the cost of PGD may decline over time as the procedure becomes more widely available, the current cost precludes access for many women. As it stands, PGD will inevitably be of greater accessibility to women in higher socio-economic classes, causing some to speculate that genetic disabilities will one day become a stigma of the lower class. Currently, insurance coverage is inadequate to sufficiently narrow this gap in access between rich and poor. Expanded insurance coverage of PGD could improve access, but it is unlikely to what extent the public is willing to devote funding – tax dollars or private funds – for procedures like PGD.

Concerns about Use
PGD is a powerful tool that may allow parents to identify and select only the embryos that possess the genetic characteristics they may desire in their children. The fundamental societal questions are whether and under what conditions PGD should be used. Many observers have raised concerns about how an increase in use of PGD will affect society. Others are concerned that PGD has the potential to fundamentally alter the way we view human reproduction and our offspring as well.

Others oppose PGD on the grounds of protecting the moral status of the embryo. For those who categorically oppose manipulation or destruction of human embryos, PGD is never appropriate because it necessarily involves one or both. But, even here, it is important to distinguish between personal preference and policy preference. Public opinion research conducted by the Genetics and Public Policy Center has shown that some people who would never themselves consider PGD do not necessarily support policy prohibiting others from using it. There are also those who may find PGD appropriate when used to detect certain serious medical conditions but have reservations about its use for other purposes.

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Resources
RHTP Position Statement on Prenatal and Preimplantation Genetic Testing and Diagnosis
[PDF, 106 KB]
RHTP Fact Sheet on PGD [DOC, 30KB]
Genetics and Public Policy Center
The Genetics and Public Policy Center has been established to be an independent and objective source of credible information on genetic technologies and genetic policies for the public, media and policymakers.
 
 
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