How to Measure Nuchal Translucency on Ultrasound
Gestational Age and Timing
Nuchal translucency must be measured between 11+0 to 13+6 weeks of gestation (or 14+0 weeks by some protocols), as the measurement loses predictive value outside this narrow window. 1, 2, 3 The optimal crown-rump length (CRL) for successful NT measurement is 45-66 mm, corresponding to 11+0 to 12+6 weeks, with success rates of 81.8-84.4% compared to only 66.9% at 13 weeks. 4
Standardized Measurement Technique
Image Acquisition Requirements
The fetus must be positioned in a true midsagittal plane showing the entire fetal profile with the spine posteriorly. 1 Key technical requirements include:
Magnification: The image should be magnified so that the fetal head and upper thorax occupy at least 75% of the screen to ensure accurate caliper placement. 1
Fetal position: The fetal neck must be in a neutral position—neither hyperextended nor hyperflexed—as both extremes can artificially increase the NT measurement. 1
Amnion distinction: The amnion must be clearly distinguished from the overlying fetal skin, as confusion between these structures is a common source of measurement error. 1
Caliper Placement
Place calipers on the inner borders of the hypo-echoic space, measuring from the outer edge of the overlying skin to the inner edge of the underlying soft tissue covering the posterior cervical spine. 1, 3 The NT is defined as the hypo-echoic (fluid-filled) space between the overlying skin and underlying soft tissues of the posterior cervical spine. 1, 3
Always record the maximum measurement obtained during the examination, as NT thickness varies with fetal movement. 1 Multiple measurements should be taken, and the largest measurement is used for risk calculation. 1
Ultrasound Approach Selection
Transabdominal Ultrasound (Primary Method)
Transabdominal ultrasound successfully obtains NT measurements in approximately 95% of patients and should be the initial approach. 1 This method provides adequate visualization in most cases and allows for greater probe mobility to obtain optimal imaging planes. 1
Transvaginal Ultrasound (Alternative Method)
Switch to transvaginal ultrasound when optimal transabdominal views cannot be obtained, which occurs in about 5% of patients. 1 Specific indications include:
- High body mass index limiting transabdominal visualization 1
- Multiple abdominal surgeries or scarring 1
- Transverse fetal lie in the lower uterus 1
- Suboptimal transabdominal views where NT borders cannot be clearly identified 1
NT measurements are equivalent between transabdominal and transvaginal approaches when the same standardized principles are applied. 1 The higher resolution of transvaginal ultrasound can more clearly identify the amnion and NT borders in challenging cases. 1
Interpretation Thresholds
An increased NT is defined as a measurement ≥3 mm or above the 99th percentile for the crown-rump length at 11-14 weeks. 1, 2, 3 This threshold is uniformly increased at any CRL between 11 and 14 weeks. 1
NT thickness increases physiologically with crown-rump length, so gestational age must be considered when determining if a measurement is abnormal. 1, 3 The relationship follows a quadratic equation, with NT increasing between 9-12 weeks and then decreasing at 13-14 weeks. 5
Quality Assurance Considerations
Measurement of NT has been standardized through quality assurance programs established by the Fetal Medicine Foundation and the Nuchal Translucency Quality Review to minimize inter- and intra-observer variability. 1, 2 There remains considerable variability in NT measurements even among trained operators, highlighting the critical importance of:
- Ongoing quality assessment and certification 1
- Adherence to standardized measurement protocols 1, 2
- Regular review of measurement technique 1
Common Pitfalls to Avoid
Confusing the amnion with fetal skin: This is the most common source of measurement error and can falsely elevate the NT measurement. 1
Measuring with the fetal neck hyperextended or hyperflexed: Both positions artificially increase NT thickness. 1
Insufficient magnification: Inadequate zoom prevents accurate caliper placement and increases measurement error. 1
Recording the minimum rather than maximum measurement: NT varies with fetal movement, and the maximum measurement must be used. 1
Measuring outside the 11-14 week window: The false-positive rate increases significantly with advancing gestational age, rising from 1.7% at 12-12.9 weeks to 15.1% at 14.0-14.9 weeks. 6
Special Considerations for Twin Pregnancies
In dichorionic twins, measure each fetus individually using standard singleton thresholds, as NT distribution does not differ significantly between singletons and twins. 1, 2, 7 Calculate fetus-specific risks independently for each twin. 1, 2
In monochorionic twins, calculate pregnancy-specific risk using the average NT thickness of both fetuses. 1, 2