Prenatal aneuploidy screening tests for cell free DNA, or small fragments of DNA found in maternal blood during pregnancy. Many of these fragments come from the placenta and can be used to test for the chances of chromosomal disorders in the fetus during pregnancy. Chromosomal disorders most commonly include trisomy 21 or Down syndrome where an extra chromosome is present. However, cfDNA (cell free DNA) can also screen for the chance of trisomy 21, 18, or 13, as well as rare genetic chromosomal disorders and extra genetic material in sex chromosomes.
How accurate is cfDNA screening?
This screening method is most accurate for diagnosing Down syndrome but less so for other trisomies (the presence of an extra chromosome) and sex chromosome disorders. Its accuracy can depend on several factors like the baseline risk of having a trisomy, which in turn can depend on factors like maternal age. The chances of having a child with Down syndrome at 40 years old is much higher than the chances at 30 years old, for example. This means cfDNA testing will be much more accurate in women near 40 who already have a higher baseline risk of having children with chromosomal abnormalities. This also means that younger women can have a higher risk of a false positive result where a high risk of chromosomal abnormalities is flagged but is not actually present.
What if I have a positive result?
CfDNA is not a comprehensive test and a positive result does not mean that a chromosome disorder is present. Rather, a positive result with this screening process tells you that you have a higher chance of a chromosomal disorder being present. This means you should first meet with your maternal fetal medicine specialist before making irreversible decisions such as ending your pregnancy. Your maternal fetal medicine specialist can assess your results and recommend further testing options such as an ultrasound and diagnostic tests like chorionic villous sampling (CVS) or amniocentesis.
What if the screening doesn’t give a result?
In cases where cfDNA screening doesn’t produce a result, this means there either isn’t enough fetal DNA available or there was difficulty interpreting the test result. Factors like taking the test before ten weeks of pregnancy, being over 250 pounds, or the child having a chromosomal abnormality are common reasons that there may be difficulty interpreting the screening. In this case, your maternal fetal medicine specialist might recommend a repeat test or another diagnostic test.
What are the limitations of cfDNA screening?
The scope of this testing process is limited, and it can only evaluate three common trisomies as well as fetal sex chromosome abnormalities. Other birth defects will require separate tests. Unfortunately, it’s less accurate in women bearing twins, particularly if one twin stopped developing early in the pregnancy. Your maternal fetal medicine specialist can recommend your best options for cfDNA screening and how it can benefit you and your pregnancy.
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At Carnegie Imaging, we offer all the tools you need to ensure you can make the best choices for the health of you and your child. To meet with our maternal fetal medicine specialists, contact our New York City offices by calling or filling out our online form.
Information for this article was provided by the Society for Maternal Fetal Medicine.
Carnegie Imaging for Women blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!