Normal Nuchal Translucency Measurement
A normal nuchal translucency (NT) measurement is defined as less than 3 mm or below the 99th percentile for crown-rump length when measured between 11+0 and 14+0 weeks of gestation. 1
Measurement Parameters and Timing
NT measurement must be performed between 11+0 and 14+0 weeks of gestation, as the measurement is crown-rump length dependent and loses predictive value outside this window. 2
The NT measurement increases physiologically with crown-rump length, so gestational age must be considered when determining if a measurement is abnormal—a single fixed cutoff cannot be applied across all gestational ages within the screening window. 1, 2
Transabdominal ultrasound can successfully obtain NT measurements in approximately 95% of patients, with transvaginal ultrasound reserved for cases where optimal views cannot be obtained transabdominally. 1
Definition of Abnormal Values
An increased (abnormal) NT is defined as a measurement ≥3 mm or above the 99th percentile for the crown-rump length. 1
Some research suggests that NT measurements between 2.5-2.9 mm may represent a "gray zone" with intermediate risk, showing a 2.5-fold increased risk of aneuploidy compared to NT <2.5 mm, though this remains below the standard clinical threshold for abnormal. 3
Historical literature has variably used cutoffs of ≥2.5 mm or ≥3 mm to define abnormal measurements, though the current standard clinical threshold is ≥3 mm. 4, 5
Clinical Context and Limitations
A normal NT measurement (below 3 mm) is reassuring but should not be used in isolation to determine aneuploidy risk, as NT assessment alone detects only approximately 70% of trisomy 21 fetuses. 1, 2
When combined with maternal serum markers (pregnancy-associated plasma protein A and free beta-hCG), first trimester combined screening detects approximately 90% of affected pregnancies at a 5% false-positive rate. 1
Most cases of congenital heart disease have normal NT thickness—less than 25% of major heart defects are detected using the 99th percentile cutoff in euploid fetuses, confirming that normal NT does not exclude cardiac anomalies. 1
Important Caveats
Standardized measurement techniques are essential for accuracy, as established by organizations like the Fetal Medicine Foundation and Nuchal Translucency Quality Review Program—the small size of NT (typically <3 mm) approximates the threshold of normal inter-examiner and intra-examiner variability. 2, 4
Normal NT measurements do not replace the need for second trimester anatomic survey, as only 50% of major fetal anomalies can be detected prior to 14 weeks with a 3-4% false-positive rate at this early gestational age. 1
In high-risk populations (such as women over 35 years), a normal NT measurement should not discourage invasive testing if otherwise indicated, as the data are unclear whether normal NT decreases the likelihood of chromosomal abnormality sufficiently in these patients. 4