Can You Have Normal Nuchal Translucency but Abnormal NIPT?
Yes, you can absolutely have a normal nuchal translucency (NT) measurement with an abnormal NIPT result—these are independent screening tests that assess different aspects of fetal risk and do not always correlate. 1, 2
Why This Discordance Occurs
Different Screening Mechanisms
- NT measures a physical finding (the fluid-filled space at the back of the fetal neck) that correlates with aneuploidy risk but detects only approximately 70% of trisomy 21 cases when used alone 1, 3
- NIPT analyzes cell-free fetal DNA in maternal blood and has a higher detection rate for trisomy 21,18, and 13 specifically, but screens a narrower range of conditions 2
- These tests are measuring fundamentally different markers, so normal results on one test do not guarantee normal results on the other 1
Clinical Reality of Discordant Results
- A fetus can have a chromosomal abnormality (detected by NIPT) without having increased nuchal fluid accumulation 1, 3
- Conversely, increased NT can occur in chromosomally normal fetuses due to cardiac defects, genetic syndromes like Noonan syndrome or 22q11.2 deletion, or other structural anomalies that NIPT cannot detect 2, 4
Management When NIPT is Abnormal Despite Normal NT
Immediate Next Steps
- Offer genetic counseling and diagnostic testing with chorionic villus sampling (CVS) or amniocentesis to confirm the NIPT result, as NIPT is a screening test that requires confirmation before making definitive clinical decisions 2, 4
- Do not rely on the normal NT measurement to provide false reassurance—proceed with diagnostic confirmation of the abnormal NIPT 4, 5
Important Limitations to Recognize
- NIPT only screens for common trisomies (21,18,13) and sex chromosome aneuploidies, missing other chromosomal abnormalities, subchromosomal deletions/duplications, and genetic syndromes 2, 4
- If diagnostic testing (karyotype and chromosomal microarray analysis) is performed, it may reveal additional findings beyond what NIPT screens for—studies show that invasive testing detects clinically significant abnormalities in 3.1% of cases with normal NIPT results when ultrasound anomalies are present 4
Critical Pitfalls to Avoid
Don't Skip the Anatomy Scan
- Even with normal NIPT and normal NT, a detailed second trimester anatomic survey at 18-22 weeks remains mandatory 2, 3
- NT screening should never replace the standard second trimester fetal survey, as sensitivity for detecting congenital anomalies at 11-14 weeks is only 50% compared to 75% at 18-22 weeks 3
Don't Assume NIPT Covers Everything
- NIPT misses structural heart defects, skeletal dysplasias, other genetic syndromes, and chromosomal abnormalities beyond the common trisomies 2, 4
- In one study of pregnancies with ultrasound anomalies, NIPT would have missed more than 15% of chromosomal abnormalities that were detected by invasive testing 5
Understand the Residual Risk
- When NT is in the 3.0-3.4 mm range (borderline increased), even with normal NIPT for common trisomies, the residual risk for missing a chromosomal aberration is approximately 1 in 21 (4.8%) 6
- This residual risk can be lowered to approximately 1 in 464 if genome-wide NIPT with 10-Mb resolution is used, but standard NIPT does not provide this level of coverage 6
The Bottom Line on Test Independence
Normal NT does not predict normal NIPT, and vice versa—they provide complementary but distinct information about fetal risk. 1, 2 When results are discordant, always proceed with diagnostic confirmation and comprehensive fetal evaluation including detailed anatomy scan and fetal echocardiography. 2, 3