Which parameters should be assessed during a nuchal translucency (NT) scan?

Parameters to Assess During NT Scan

During a nuchal translucency (NT) scan at 11-14 weeks gestation, you must measure the NT thickness paired with crown-rump length (CRL), and should additionally assess nasal bone presence, and in experienced centers, evaluate for early structural anomalies including cardiac anatomy, brain, abdominal wall, and limbs 1.

Core Mandatory Parameters

Primary Measurements

• Nuchal Translucency Thickness: The maximum measurement of the subcutaneous fluid collection between the skin and soft tissue overlying the cervical spine, measured in the mid-sagittal plane with the fetus in neutral position 2, 1

  • Normal threshold: <3.0 mm or <99th percentile for CRL 1
  • Requires strict technical standards for accuracy

• Crown-Rump Length (CRL): Essential for gestational age confirmation and NT interpretation, as NT values are CRL-dependent 2, 1

  • Valid measurement range: 45-85 mm (corresponding to 11-14 weeks)
  • Must be obtained simultaneously with NT for proper risk calculation

Additional First-Trimester Aneuploidy Markers

• Nasal Bone Assessment: Presence or absence significantly improves aneuploidy detection rates when incorporated into screening algorithms 1
  • Absence of nasal bone is a powerful marker for chromosomal abnormalities
  • Best visualized with high-resolution imaging, potentially requiring transvaginal approach 1

Extended Assessment Parameters (When Indicated)

Cardiac Evaluation

When NT ≥3.0 mm, additional cardiac assessment should include 1:

• Basic cardiac anatomy: Four-chamber view, outflow tracts (detectable in 50-65% of major cardiac anomalies at this gestational age) 1
• Tricuspid valve flow: Assess for significant tricuspid regurgitation using color and pulsed Doppler
  • Significant if retrograde flow extends >1/3 of right atrial diameter or duration >50% of ventricular systole 1
• Ductus venosus flow: Evaluate for reversed a-wave, which increases aneuploidy risk 10-fold 1

Critical caveat: First-trimester cardiac imaging has limitations—only 50% of major anomalies detectable at 11-14 weeks versus 75% at 18-22 weeks. All cases with increased NT require repeat second-trimester fetal echocardiogram regardless of first-trimester findings 1.

Early Anatomic Survey (Experienced Centers Only)

When increased NT is identified, assess for 1:

• Brain structures: Early detection of major brain malformations
• Abdominal wall: Gastroschisis, omphalocele
• Limbs: Major skeletal abnormalities
• Frontomaxillary and iliac angles: Additional aneuploidy markers 1

Important limitation: Sensitivity for congenital anomalies is only ~50% at 11-14 weeks compared to 75% at 18-22 weeks 1, so normal early anatomy does not replace standard second-trimester anatomic survey.

Technical Quality Assurance Parameters

The laboratory must validate sonographer competency by reviewing 2:

• Paired NT/CRL measurements from at least 30 pregnancies
• Median NT MoM should fall between 0.9-1.1 MoM
• Ongoing epidemiological monitoring of each sonographer's measurements

Clinical Action Thresholds

NT ≥3.0 mm triggers immediate management 1:

1. Genetic counseling and offer invasive diagnostic testing (CVS or amniocentesis)
2. Consider cell-free DNA testing, though this misses conditions like Noonan syndrome and 22q11.2 deletion
3. Targeted genetic studies including microarray analysis recommended (10% of euploid fetuses with NT ≥3.0 mm have Noonan syndrome variants) 1
4. Second-trimester fetal echocardiogram mandatory
5. Second-trimester anatomic survey mandatory

NT 2.5-3.4 mm: Increased surveillance warranted, as adverse outcomes occur in 31.9% of cases 3

Twin-Specific Considerations

• Dichorionic twins: Assess each fetus individually; calculate fetus-specific risks using each twin's own NT 1
• Monochorionic twins: Calculate pregnancy-specific risk using average NT of both fetuses 1
• Standard thresholds apply to twins as to singletons 1

Common Pitfalls to Avoid

1. Do not use NT measurement alone for risk assessment—it detects only 70% of trisomy 21 cases in isolation 1
2. Do not skip second-trimester follow-up even if NT normalizes—risks remain elevated 1
3. Do not rely on Doppler studies in routine low-risk screening (theoretical thermal risk to developing fetus) 1
4. Do not assume normal NT excludes cardiac disease—only 25% of major heart defects show increased NT 1