Absent Nasal Bone in Fetus

Absent Nasal Bone

Absent nasal bone (ANB) is one of the strongest prenatal soft markers for aneuploidy, and unlike hypoplasia, its significance is largely independent of ethnicity.

What does “absent nasal bone” mean?
• True non-visualization of the nasal bone on a correct mid-sagittal view
• Skin over the nose is visible, but no echogenic ossified nasal bone

A nasal bone absent in the first trimester may appear later, but persistent absence increases concern.
Critical distinction

Ethnicity affects hypoplastic nasal bone far more than absent nasal bone.

Population data (approximate)
• Prevalence of absent NB in euploid fetuses: ~0.1–0.5%
• Minor ethnic variation exists, but predictive value remains high

  Ethnicity   Effect on significance
  South Asian   Slightly higher prevalence, but still abnormal
  East Asian   Same
  African   Same
  Caucasian   Same

Absent nasal bone is concerning in all ethnic groups.

A. Aneuploidy (primary association)
Trisomy 21 (Down syndrome)
• Strongest link
• Absent NB seen in:
o ~60–70% in first trimester
o ~40–50% in second trimester
• Likelihood ratio: ~20–30

Pathogenic copy number variants if associated with:
• Abnormal facies
• Cardiac defects
• CNS anomalies

• 22q11.2 deletion
• Wolf–Hirschhorn syndrome (4p–)
• Cri-du-chat (5p–)

Investigations (what to offer)
First-line
• Invasive testing recommended
• Chromosomal microarray preferred over karyotype

Role of NIPT
• Acceptable if parents decline invasive testing
• Not definitive in presence of ANB

Key take-home points
• Absent nasal bone is one of the strongest soft markers
• Ethnicity does not neutralize its significance
• Isolation does not equal reassurance
• Invasive genetic testing should be offered
• Prognosis depends on the underlying diagnosis

First-tier ultrasound parameters
A. Nuchal markers

  Parameter   Why it matters   Red flag
  NT (1st trimester)   Strongly additive risk with absent NB   NT ≥95th centile
  Nuchal fold (2nd trimester)   Most powerful soft marker   ≥6 mm
  Generalized skin edema   Suggests aneuploidy or hydrops   Any diffuse thickening

B. Facial profile assessment

  Parameter   Interpretation
  Midface hypoplasia   Strong T21 association
  Flat facial profile   Seen in T21, some skeletal dysplasias
  Premaxillary protrusion   Supports T21
  Retrognathia / micrognathia   Think T18, triploidy, syndromes

Second-tier structural survey
A. Cardiac evaluation

  Finding   Significance
  AVSD   Strong T21 marker
  VSD (esp inlet)   Additive risk
  Outflow tract anomalies   Syndromic suspicion
  Abnormal ductus / arch   Think T22q11, T18

B. CNS markers

  Marker   Suggests
  Ventriculomegaly   T21, T18
  Enlarged CSP   T21
  Absent / hypoplastic CC   Syndromic
  Posterior fossa anomaly   T18, Joubert spectrum

C. Limbs and skeleton

  Parameter   Why
  Short femur/humerus   Classic T21 marker
  5th finger clinodactyly   T21
  Overlapping fingers   T18
  Foot–FL ratio   Excludes skeletal dysplasia

3. Placental and growth context

  Parameter   Interpretation
  Early FGR   T18, triploidy
  Abnormal placental morphology   Triploidy
  Oligohydramnios   T18, renal syndromes
  Polyhydramnios   T21 (late), GI obstruction

4. Doppler modifiers

  Doppler   Meaning
  Absent/reversed DV a-wave (1st trimester)   High aneuploidy risk
  Abnormal tricuspid flow   T21
  Normal Dopplers   Risk reduction in isolated cases

Risk stratification summary (console-ready)

Low risk
• Isolated absent NB
• Normal NT/NF
• Normal cardiac + anatomy
• Low-risk NIPT

Intermediate risk
• Absent NB + 1 additional soft marker
• Normal anatomy otherwise

High risk
• Absent NB +
o Increased NT/
o Cardiac defect
o Multiple soft markers
o Structural anomaly