Noninvasive prenatal screening (NIPS) that uses cell-free DNA (cfDNA) from the plasma of pregnant women offers tremendous potential as a screening method for fetal aneuploidy. ACOG and SMFM recommended it as a screening option for women at increased risk of aneuploidy such as AMA, or women with positive first or second trimester screening test results.
cfDNA of origin arises primarily from the placenta. It is 3-13% of total cell-free maternal DNA. This is the fetal fraction number you see on many of the lab reports. You usually need a fetal fraction of at least 4% to get a result. Testing can be performed from 9 or 10 weeks until delivery. Results take 7-10 days. Cell-free DNA has a Positive Predictive Value (PPV) for Down syndrome of 93%, Trisomy 18 64%, Trisomy 13 44%, Turners 39%, and Sex Determination 98%. Accuracy in multiple gestations is limited. The residual risk of a chromosomal abnormality with a normal cell-free DNA result after an abnormal traditional screening test has been shown to be 2%. So, consider genetic counseling to offer the option of invasive testing.
All positive cfDNA screening tests should have diagnostic testing to confirm the results. Management decisions, including pregnancy termination, should not be based on cell-free DNA screening alone. Women whose results are not reported indeterminate or uninterpretable (a “no call”) should receive further counseling and offered diagnostic testing because of the increased risk of aneuploidy (20%). Routine of cfDNA screening for microdeletion syndromes, per ACOG, should not be performed. Patients should be counseled that a negative cell-free DNA test result does not ensure an unaffected pregnancy.
ACOG states conventional screening methods remain the most appropriated choice for first line screening for most women in the general OB populations. This is because cfDNA provides lower detection rates than sequential screening if considering detection of all chromosomal abnormalities, not just Down syndrome.
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!