What are the indications for nuchal translucency (NT) ultrasound?

Indications for Nuchal Translucency Ultrasound

Nuchal translucency (NT) ultrasound is indicated as a first-trimester screening tool for fetal aneuploidy (particularly Down syndrome), genetic syndromes, structural anomalies, and risk of intrauterine demise in pregnancies between 11+0 and 14+0 weeks of gestation. 1, 2

Primary Screening Indication

• Aneuploidy screening is the principal indication, as NT measurement combined with maternal age detects over 75% of trisomy 21 cases and 81% of all aneuploidies 1, 3
• The test functions as a powerful independent marker for chromosomal abnormalities including trisomies 21,18, and 13, Turner syndrome, and triploidy 1, 2
• NT ≥3 mm carries approximately a 33% risk of chromosomal abnormalities, with half being trisomy 21 2

Specific Clinical Scenarios Warranting NT Evaluation

High-Risk Populations

• Advanced maternal age (≥35 years) or elevated age-related risk 1
• Prior pregnancy with chromosomal abnormality or family history of genetic disorders 1
• Positive first-trimester combined screening (NT plus biochemical markers) 1

Follow-up Indications

• Dichorionic twin pregnancies benefit from NT screening, particularly when one or both fetuses have NT ≥3 mm, which warrants additional Doppler evaluation and fetal echocardiography 1
• Borderline NT measurements (2.5-2.9 mm) increase aneuploidy risk 2.5-fold compared to NT <2.5 mm, justifying screening in this population 4

Optimal Timing and Technical Requirements

• Schedule at 12-13 weeks' gestation based on last menstrual period to minimize missed miscarriages (which decrease with advancing gestation) while avoiding crown-rump length >84 mm (which increases with advancing gestation and prevents accurate measurement) 5
• The measurement window is strictly 11+0 to 14+0 weeks, as NT is crown-rump length dependent and loses predictive value outside this range 1, 2
• Standardized measurement technique following Fetal Medicine Foundation or Nuchal Translucency Quality Review criteria is essential for accuracy 2, 6

Common Pitfalls to Avoid

• Do not rely on NT measurement alone: It detects only 70% of trisomy 21 cases when used as a standalone test and should be combined with maternal age and/or biochemical markers 2
• Recognize technical limitations: Success rates for obtaining adequate measurements are only 58% overall, improving to 74% at ≥10 weeks' gestation 7
• Understand the limitations of early anatomic survey: Even when performed at 11-14 weeks in experienced centers, sensitivity for detecting congenital anomalies is only 50% compared to 75% at 18-22 weeks 1, 2

Management When NT is Increased (≥3 mm)

When NT measures ≥3 mm, the following cascade of evaluations is indicated 1, 2:

• Immediate genetic counseling and offer of invasive diagnostic testing (amniocentesis or chorionic villus sampling)
• Cell-free fetal DNA screening as an alternative, though this only detects trisomies 21,18,13 and sex chromosome aneuploidies, missing conditions like Noonan syndrome and 22q11.2 deletion syndrome 1
• Targeted genetic studies including microarray analysis, as 10% of euploid fetuses with NT ≥3 mm have genetic variants consistent with Noonan syndrome 1
• Second-trimester anatomic survey at 18-22 weeks regardless of genetic testing results 1
• Fetal echocardiography due to elevated risk of congenital heart disease even in euploid fetuses 1, 2
• Serial fetal surveillance throughout pregnancy to monitor growth and well-being, as intrauterine demise risk increases proportionally with NT thickness 1

Transvaginal Approach Considerations

• Transvaginal ultrasound may be advantageous in patients with increased body mass index, abdominal wall scarring, or fetal positioning in the lower uterus to ensure accurate measurement 1
• Measurements are equivalent between transabdominal and transvaginal approaches when proper technique is applied, so transvaginal imaging is not required if transabdominal views are satisfactory 1