What is the significance of an absent nasal bone in a second trimester fetus with a normal nuchal translucency (NT) scan?

Significance of Absent Nasal Bone in Second Trimester Fetus with Normal Nuchal Translucency

An absent nasal bone in a second trimester fetus with normal nuchal translucency indicates an increased risk for trisomy 21 that warrants counseling and offering further aneuploidy evaluation, even with normal NT findings. 1

Risk Assessment and Clinical Significance

Risk of Aneuploidy

• Absent nasal bone is strongly associated with trisomy 21 and other chromosomal abnormalities
• When considered as an isolated marker (with normal NT):
  • Positive likelihood ratio (LR) is 6.6 for trisomy 21 1
  • When combined with other markers or abnormalities, the LR increases to 23 1

Ethnic Variation

• Important consideration in risk assessment:
  • Occurs in 0.1-1.2% of euploid pregnancies 1
  • Higher prevalence (9%) in Afro-Caribbean populations 1, 2
  • Less common in White and Asian populations 1

Management Algorithm

For Patients with No Previous Aneuploidy Screening:

1. Provide counseling about increased risk of trisomy 21 1
2. Offer options for evaluation:
   • Non-invasive screening via cell-free DNA (cfDNA) testing (preferred)
   • Quadruple screen if cfDNA unavailable or cost-prohibitive
   • Diagnostic testing via amniocentesis
   • Decision depends on clinical circumstances and patient preference

For Patients with Previous Negative Serum Screening:

1. Recalculate risk incorporating the absent nasal bone finding
2. Discuss options:
   • No further evaluation
   • Non-invasive screening via cfDNA
   • Diagnostic testing via amniocentesis
   • The high LR may increase risk above threshold for further evaluation

For Patients with Previous Negative cfDNA Screening:

• No further aneuploidy evaluation recommended 1
• In this context, absent nasal bone is likely a normal variant

Additional Considerations

Comprehensive Anatomic Evaluation

• Careful evaluation of fetal anatomy is warranted as absent nasal bone can be associated with:
  • Other genetic syndromes beyond trisomy 21 1
  • Multiple congenital anomalies in some cases 3

Prognostic Implications

• When isolated (no other markers or anomalies) and aneuploidy is excluded:
  • Generally good prognosis 3
  • Normal newborn examination findings provide reassurance 3

Common Pitfalls

1. Failing to consider ethnic variation - higher prevalence in Afro-Caribbean populations may affect positive predictive value 2
2. Not evaluating for additional markers - presence of other markers significantly increases risk 4
3. Overreliance on normal NT - absent nasal bone remains significant even with normal NT 1
4. Inadequate imaging technique - proper midsagittal plane visualization is essential for accurate assessment 1

Conclusion for Clinical Practice

The finding of an absent nasal bone in the second trimester, even with normal nuchal translucency, represents a significant soft marker for trisomy 21 that should prompt further evaluation. The management approach should be based on previous screening history, with cfDNA testing being the preferred next step for those without prior screening or with previous serum screening. Proper counseling about the increased risk and options for further evaluation is essential.