Pre-Implantation Genetic Diagnosis (PGD)
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One cell is removed from an 8-celled embryo for genetic testing.

Genetic Testing Of Embryos

Contents

What Is Pre-Implantation Genetic Diagnosis?
What Does PGD Test For?
What Is The Difference Between PGD and PGS?
What Is The Procedure For Pre-Implantation Genetic Diagnosis?
What Are The Benefits Of PGD?
What Are The Limitations Of PGD?
What Ethical Issues Does PGD Raise?
How Much Does PGD Cost?



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Genetic Testing

Terminology: Pre-implantation genetic diagnosis (PGD) is also known as pre-implantation genetic testing (PGT).

What Is Pre-Implantation Genetic Diagnosis?

Pre-implantation genetic diagnosis (PGD) is a service offered by some fertility clinics to couples undergoing IVF. PGD makes it possible to check an embryo for chromosome abnormalities and gender before implanting in the mother. If you carry a genetic disease or gender-linked disease a counselor may recommend having your embryo's chromosomes assessed before having them transferred to your womb. PGD can be done to check the embryo has the correct amount of genes or to check for the presence of a certain gene. In most cases both partners have been genetically screened and one or both have been identified to be carriers of genetic abnormalities.

Related Questions
How important is genetic screening for IVF babies?

What Does PGD Test For?

PGD can test for:-

Down Syndrome and other common trisomies (where an extra gene is present) such as trisomy 13 and 18.
• Inherited genetic diseases such as Huntington's disease, cystic fibrosis and sickle cell anemia.
• Gender-related disorders such as hemophilia (more common in boys), fragile X syndrome and most neuromuscular dystrophies.
• Anomalies in the Y and X chromosomes, such as Turner syndrome and Klinefelter.

What Is The Difference Between PGD and PGS?

PGD may be recommended where either or both parents carry a genetic abnormality and doctors want to test for this disorder in the embryo before it is implanted. Preimplantation genetic screening (PGS) is where both parents are assumed to be chromosomally normal, but they want the embryo screened for abnormalities, just to be safe (the mother may be aged over 35 and is worried about the risk of Down syndrome for example). Both tests are an alternative to genetic testing during pregnancy, where procedures such as amniocentesis or chorionic villus sampling (CVS) are used. Of course testing before the embryo is implanted is better because it eliminates the dilemma of a termination if the prenatal test is positive.

What Is The Procedure For Pre-Implantation Genetic Diagnosis?

Using IVF technology, one cell called a blastomere is removed from an 8-celled embryo. The cell is then tested for number of chromosomes and specific genetic defects. PGD is time intensive and it is estimated that up to 10 percent of diagnoses are not correct. This is because only one cell is being tested, compared to hundreds via an amniocentesis test. It is further complicated by the fact that some embryos have something called mosaicism. This means that not all cells have the same number of chromosomes. So while the cell being tested may be normal, all the other cells might be abnormal (or vice versa). Only certain fertility clinics are equipped to carry out PGD. The test itself is performed by an embryologist.

What Are The Benefits Of PGD?

The PGD procedure:

• Is performed before implantation so reduces the need for amniocentesis in pregnancy.
• It can detect abnormalities before pregnancy, allowing couples to decide if they want to continue or not with implantation.
• Allows couples who could not have children for fear of passing on genetic disorders to have biological children.
• It may help reduce costs associated with children born with birth defects.

What Are The Limitations Of PGD?

• It does not test for all chromosome abnormalities, just the most common ones. It misses 10 to 15 percent of abnormalities.
• In the United States only one cell is usually tested, although some centers test 2 cells. Removing 2 cells remains controversial - removing one cell hurts the embryo a little bit, removing 2 may damage it much more. At this time it is not clear how much damage removing just one cell to an embryo does (although it is thought to be relatively safe).
• Sometimes one cell can develop differently to the other cells, even though at such an early stage they should be identical. Mosaicism is where one cell has chromosomes that differ to the others. Up to 50 percent of all embryos can have mosaicism. Mosaicism in a tested cell can result in a false diagnosis - either false positive or false negative result.
• Some studies show that embryos with suspicious chromosome activity at time of testing (day 3) can self-correct by day 5. As such, much more research is needed into the mysteries of embryo growth.
• PGD does not increase IVF pregnancy success rates.

What Ethical Issues Does PGD Raise?

Those worried about PGD testing claim it allows couples to have designer babies. PGD testing allows parents to choose the sex of their baby and to select healthy, non-genetically diseased embryos. But where does it end? Many ethicists worry parents in the near-future will be able to design their perfect baby, by selecting physical appearance of their child, including eye and hair color and height. Inevitably it is feared that the rich (those that can afford the procedure) will become like some super race of America’s top models, while the rest of the population become stigmatized for their ordinary or sub-standard physical appearance. Another concern is that PGD leads to the destruction of rejected embryos, but in practice, not that many are destroyed.

How Much Does PGD Cost?

The average cost of PGD is $3,200 in the United States. The fee for this optional service will be tagged on to your overall cost for IVF treatment. Most insurance companies do not cover the costs.

  Related Articles on ART Procedures

For more information, see the following:

Genetic testing before pregnancy: Assessing your risk as parents.
Extracting sperm for IVF: When is it recommended? Sperm aspiration.
Embryo donation: Receiving help from another couple.
Donor cytoplasm: IVF treatment, read why it's banned.
Egg freezing: How scientists can freeze your eggs.

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