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Confirmatory Prenatal Diagnosis following Preimplantation Genetic Diagnosis (PGD)

Introduction

Preimplantation genetic diagnosis (PGD) is a technique that enables couples at genetic risk to have their embryos screened before a pregnancy is established.  PGD can be used to screen for known single gene disorders, translocations, and some of the more common chromosomal abnormalities such as Down Syndrome.

You should be aware that the results obtained from PGD are NOT 100% accurate.  At best, the accuracy of the test is approximately 90% if you have had PGD for chromosomal screening or up to 98% if you have had PGD for a specific genetic condition.  Consequently, there may be up to a 10% error rate associated with any test performed.

What factors can influence the accuracy of PGD?

The accuracy of the result can be affected by a number of different factors, including:

• The cell biopsied from the embryo.  It is possible that the cell biopsied is NOT representative of the entire embryo.  This may lead to a misdiagnosis.
• The number of cells biopsied from the embryo.  The accuracy is higher if results are obtained from two cells rather than one cell. 
• The clarity of results.  Some PGD results are more difficult to interpret than others.  If the results are not clear, we may report a reduced accuracy which may differ from that given in the feasibility report.

In addition, when using linked markers to increase the accuracy of PGD for single gene disorders, it is possible that genetic recombination can occur in 1 to 2% of cases.  Genetic recombination can occur when the DNA surrounding the gene of interest swaps with DNA on another chromosome.  This can result in a misdiagnosis.  As first outlined by your IVF doctor, following a positive pregnancy result from a PGD cycle, confirmatory prenatal diagnosis is strongly recommended.  Prenatal diagnosis has greater than 99% accuracy as thousands of cells are available for testing.

What to do next?

Prenatal diagnosis is generally performed at 11 to 13 weeks gestation for Chorionic Villus Sampling or 15 to 19 weeks gestation for amniocentesis.  The testing may be able to detect other chromosomal abnormalities that were not possible to detect using the specific PGD test applied to your embryos.
 
As Monash IVF does not routinely offer prenatal diagnosis testing, we recommend that you contact the Genetic Counsellor at Monash IVF on +61 03 9590 8336 or the Genetic Counselling Service in your State to discuss the option of pursuing confirmatory prenatal testing. This is an important step to determine if prenatal testing is the right choice for you.

Quality Systems

Monash IVF employs a very high standard of quality assurance. The laboratories are NATA accredited.  Through the application of quality systems we monitor and provide standards of excellence in quality service, care and advice.