Genetic testing without an amnio?

July 05, 2013 12:00 AM

Q) I’ve heard there’s now genetic testing that doesn’t involve the risks of an amnio. What are the pluses and minuses to alternative screenings?

 

A) There are different screening tests available to determine your risk for specific genetic conditions during pregnancy. Most screening tests detect the risk for common trisomies, which are conditions that involve the presence of an extra chromosome. Down syndrome (trisomy 21) is an example of this type of condition. Unlike an amniocentesis, which involves the possible risk of miscarriage, these screening tests pose no risk to the mother or pregnancy. It is important to discuss these testing options with your physician or genetic counselor to ensure the test selected is appropriate given your specific pregnancy considerations.

 

Some screening methods, like NT or nuchal translucency testing, involve a specialized ultrasound that combines a measurement of the back of the fetal neck with your maternal blood sample. This screening may only be done in the first trimester or may combine information from blood samples taken during both first and second trimesters. These screening tests estimate the risk for the pregnancy to have chromosomal conditions with a detection rate for Down syndrome of approximately 85–95%.

 

The newest type of screening is called non-invasive prenatal testing (NIPT) or cell free DNA testing and provides more accurate results than the screening tests described above. This safe screening method involves a blood sample from the mother.  There are freely floating pieces of DNA from the pregnancy in the mother’s bloodstream. These pieces of DNA are isolated, sequenced and counted to determine if there are more than expected. The detection rate for Down syndrome with NIPT, like the Harmony Prenatal Test, is >99% and the false positive rate is significantly lower at <0.1%.

 

It is important to remember that screening tests are not diagnostic.  As such, it is recommended that patients who receive an increased risk from a screening test consider diagnostic testing through chorionic villus sampling or amniocentesis.  Additionally, screening tests only look for specific chromosome conditions.  Diagnostic testing is recommended for women who desire a comprehensive genetic evaluation of the pregnancy.

 

By Dr. Thomas Musci, Vice President, Clinical Development and Medical Affairs of Ariosa Diagnostics

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