What should I do if the nasal bone is absent on a first-trimester nuchal translucency (NT) scan?

Management of Absent Nasal Bone on First-Trimester NT Scan

When the nasal bone is absent on a first-trimester NT scan, you should immediately offer genetic counseling and proceed with either cell-free DNA (cfDNA) testing or invasive diagnostic testing (CVS/amniocentesis), with the specific pathway determined by the nuchal translucency measurement. 1

Risk Stratification Based on NT Measurement

The absence of the nasal bone is a powerful marker for aneuploidy, but your management algorithm depends critically on the NT thickness:

NT ≥ 3 mm (or ≥99th percentile)

• Offer immediate genetic counseling and invasive diagnostic testing (CVS or amniocentesis) as first-line 1
• Do not wait for maternal serum results—approximately one-third of fetuses with NT ≥3 mm will have chromosomal abnormalities 1
• If the patient declines invasive testing, cfDNA is an acceptable alternative, though it will miss important genetic syndromes 1
• Critical caveat: Even if cfDNA returns low-risk, you must still recommend chromosomal microarray analysis (CMA) after karyotyping, as 9.4% of this group has pathogenic copy number variants detectable only by CMA 2

NT between 95th-99th percentile

• Offer karyotyping as first-line procedure 2
• In this group, 57% have major chromosomal anomalies when nasal bone is absent 2
• cfDNA alone is insufficient—it misses conditions like Noonan syndrome and 22q11.2 deletion syndrome 1

NT <95th percentile

• Offer cfDNA testing or invasive testing based on patient preference 3, 2
• Risk is lower (5% chromosomal abnormalities) but still significantly elevated compared to general population 2
• Genetic counseling remains essential to discuss limitations of cfDNA

Why Invasive Testing Matters

cfDNA screening only detects trisomies 21,18,13, and sex chromosome aneuploidies—it will miss:

• Noonan syndrome (present in 10% of euploid fetuses with NT ≥3 mm) 1
• 22q11.2 deletion syndrome 1
• Other pathogenic microdeletions/microduplications 2
• Triploidy 4

This is why the guidelines emphasize that failure to undergo invasive diagnostic testing means missing certain genetic causes of absent nasal bone 1.

Required Follow-Up Regardless of Genetic Testing Results

Even if genetic testing returns normal, absent nasal bone with any degree of NT elevation requires:

1. Second-trimester fetal echocardiogram (18-22 weeks)

   • Mandatory regardless of karyotype results 1
   • Congenital heart disease risk remains elevated even in euploid fetuses 1
   • First-trimester echo can be performed but must be repeated in second trimester due to 50-65% detection rate limitations 1

2. Detailed anatomic survey (18-22 weeks)

   • Required for all cases continuing pregnancy 1
   • First-trimester anatomy scan only detects 50% of anomalies vs. 75% at 18-22 weeks 1

3. Serial fetal surveillance in third trimester

   • Monitor growth and well-being 1
   • Risk of intrauterine demise proportional to NT enlargement 1

Evidence Quality Note

The ACR Appropriateness Criteria [1-1 provide the most comprehensive and recent guideline framework (2020), emphasizing that absent nasal bone is useful for patient counseling but the NT measurement ≥3 mm with or without additional markers warrants further evaluation 1. Research data 2 from 2020 confirms that genetic syndrome risk relates mainly to NT thickness rather than absent nasal bone alone, with CMA pathogenic variants found only in the NT >99th percentile group.

Common Pitfall to Avoid

Do not rely solely on cfDNA screening when nasal bone is absent—this is the single most important error to avoid. While cfDNA has excellent sensitivity for common trisomies, it creates false reassurance for the genetic syndromes and structural anomalies that absent nasal bone may herald. The 2025 ACMG guidelines 3 recommend cfDNA for general population screening, but this recommendation assumes normal first-trimester ultrasound findings.