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How is Trisomy 18 diagnosed?

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Carrie, pregnant with Zachary

A lot of prenatal testing is available which may indicate Trisomy 18. It is important to understand that there are two types of testing: screening and diagnostic.

Screening tests indicate a risk, or likelihood that Trisomy 18 is present. These tests take the results of everyone who has had the same testing, and they compare your specific results with that group. Then they use statistics to identify the odds that it is present in your child, based upon the number of times others with the same test results have had children with Trisomy 18 in the past.

This is much the same way that weather is forecast, by saying there is a 20% chance of rain because 20% of the time, when the conditions were the same, it has rained. Just as the weather forecast is not completely accurate, screening tests are not a diagnosis but only an indication that the risk is higher than normal.

The following are screening tests, which CANNOT diagnose Trisomy 18 (click on the link for detailed information about the test):

AFP (also known as triple screen, quad screen, maternal serum screening)

Ultrasound (standard, level II, level III, 3D)

Diagnostic tests check actual cells and can determine if Trisomy 18 is actually present. This is a diagnosis, since the condition has actually been found in the cells.

The following are diagnostic tests, which CAN diagnose Trisomy 18. (click on the link for detailed information about the test)

CVS (Chorionic Villi Sampling)

Amnio (Amniocentesis, FISH test)

Knowing which test was used is important in deciding what your next steps are.

If you already have a diagnosis, what next? See Making Decisions After a Prenatal Diagnosis.


More reading about prenatal testing options:

NIH FASTER Study Concludes First Trimester Screening is Better Than Second Trimester Down Syndrome Screening
First trimester Nuchal Translucency/freeBeta/PAPP-A prenatal screening for chromosomal disorders (Downs, T18/T13) has been confirmed by the NIH FASTER Study to be more effective than second trimester screening methods.

Information on Accuracy of Maternal Serum Screening Test (AFP, Triple Screen)
What are my chances of having a child with Down syndrome, trisomy 18 or an open neural tube defect if I receive a screen-positive result?


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