No, Nuchal Translucency and Nuchal Skin Fold Thickness Are Not the Same
Nuchal translucency (NT) and nuchal skin fold thickness (NSFT or NF) are distinct measurements performed at different gestational ages that assess different anatomical structures and serve as independent markers for chromosomal abnormalities, particularly Down syndrome. 1
Key Differences
Timing of Measurement
- NT is measured between 11+0 to 14+0 weeks of gestation during the first trimester, as the measurement is crown-rump length dependent and loses predictive value outside this narrow window 2, 3, 4
- NF is measured between 14-28 weeks of gestation during the second trimester, typically at the time of the anatomic survey 5, 1
Anatomical Definition
- NT measures the hypoechoic (echo-free) space between the overlying skin and underlying soft tissues of the posterior cervical spine in the first trimester fetus 3, 4, 6, 7
- NF measures the actual thickness of the nuchal skin fold itself in the second trimester 5, 1
Measurement Thresholds
- NT is considered abnormal when ≥3 mm or above the 99th percentile for crown-rump length 2, 3, 4
- NF is considered abnormal when ≥6 mm at 14-21 weeks' gestation 5
Statistical Independence
Research demonstrates no statistically significant correlation between NT and NF measurements in either Down syndrome (r = 0.076, P = 0.76) or unaffected pregnancies (r = -0.021, P = 0.76), confirming these are independent determinants of Down syndrome risk. 1 This means:
- A fetus can have increased NT in the first trimester but normal NF in the second trimester, or vice versa 1
- Both markers independently predict chromosomal abnormalities and should be considered separate screening tools 1
- The median NF value in Down syndrome fetuses is 1.538 MoM compared to 0.990 in unaffected fetuses (P < 0.0001) 1
Clinical Implications
Screening Performance
- NT alone detects approximately 70% of trisomy 21 cases, meaning 30% of Down syndrome fetuses will have normal NT measurements 2, 3, 4
- NF measurement identifies Down syndrome cases that may have had normal NT in the first trimester, as the two markers are independent 1
- First trimester combined screening (NT + PAPP-A + free β-hCG) detects approximately 90% of affected pregnancies with a 5% false-positive rate 2, 8, 4
Sequential Screening Strategy
- Gaussian modeling predicts a 91% detection rate of Down syndrome for a 5% false-positive rate when NF replaces second-trimester biochemistry in a sequential contingent screening strategy 1
- This approach leverages the independence of NT and NF measurements to maximize detection across both trimesters 1
Common Pitfall to Avoid
Never assume that a normal NT measurement in the first trimester eliminates the need for second trimester anatomic survey or NF assessment, as these are independent markers and NT screening should never replace the standard second trimester anatomic fetal survey 2, 8, 4. Even with reassuring first trimester results, the second trimester evaluation remains mandatory because the sensitivity of ultrasound at 11-14 weeks for detecting congenital anomalies is only about 50% compared to 75% at 18-22 weeks 3, 4.