Hypoplastic Nasal Bone

Hypoplastic nasal bone?

  • Underdeveloped nasal bone, visualized but smaller than expected for gestational age
  • Distinct from absent nasal bone (stronger risk marker)
  • Assessed mainly in the first and second trimesters

Embryology and biology

  • Nasal bone ossification begins at ~10–11 weeks
  • Ossification is delayed in some chromosomal conditions due to altered neural crest development
  • Also shows ethnic and familial variation

Hypoplastic = < 2.5th percentile

Nasal bone length shows significant population variation due to differences in:

  • Facial skeletal proportions
  • Timing of ossification
  • Genetic background

Because of this, a "short" nasal bone by Western reference charts may be physiologically normal in many populations.

Ethnicity-specific prevalence patterns

Ethnic group Hypoplastic NB prevalence Absent NB prevalence Interpretation
South Asian High Low Hypoplasia often normal variant
East Asian High Low Similar to South Asian
African / Afro-Caribbean High Low Absence more specific than hypoplasia
Caucasian Lower Higher specificity Both findings more predictive

Practical interpretation

Finding South / East Asian African Caucasian
Isolated hypoplastic NB Usually benign Usually benign Mild risk increase
Hypoplastic NB + normal NIPT Reassuring Reassuring Reassuring
Absent NB Concerning Concerning Highly concerning
Hypoplasia + ↑ NT / other markers Meaningful risk Meaningful risk Meaningful risk

For South Asian patients

  • "In our population, a smaller nasal bone is common and often normal."

For Caucasian patients

  • "A small nasal bone can slightly increase risk"

"The nasal bone is visualized but measures below population-based reference ranges. In isolation, this finding is common in this ethnic group and of limited clinical significance."

  • Ethnicity affects prevalence, not pathology
  • Hypoplasia is far more ethnicity-dependent than absence
  • Context always overrides measurement
  • Absent NB remains a red flag in all populations

In South and East Asian populations, an isolated hypoplastic nasal bone with normal screening is usually a normal variant.

Combined soft-marker risk recalculation examples

Scenario Baseline risk (after screening) Soft markers Ethnicity Approx. combined LR Recalculated risk Clinical interpretation Counseling takeaway
1. Isolated hypoplastic NB 1 in 1500 Hypoplastic NB only South Asian ~1–1.5 ~1 in 1000–1200 Low risk Common normal variant in this population
2. Hypoplastic NB + EIF 1 in 1200 Hypoplastic NB + EIF South Asian ~2–2.5 ~1 in 480–600 Low–intermediate Two weak markers do not equal high risk
3. Hypoplastic NB + ↑ NT (95–99%) Age-based Hypoplastic NB + increased NT Any ~16–30 High High risk Combination, not hypoplasia alone, drives risk
4. Absent nasal bone (isolated) 1 in 600 Absent NB Any ~20–30 ~1 in 20–30 High Absence is significant regardless of ethnicity
5. Hypoplastic NB + short femur NIPT low risk (<1:10,000) Hypoplastic NB + short FL Any Negligible Remains very low Low risk NIPT outweighs soft markers
6. Hypoplastic NB + structural anomaly Any Hypoplastic NB + AVSD (example) Any Not applicable High High genetic risk Structural anomaly dominates risk

Hypoplastic nasal bone + abnormal facies = not a soft-marker scenario anymore.

"Abnormal facies" usually mean on ultrasound?

  • Flat facial profile
  • Depressed or flat nasal bridge
  • Midface hypoplasia
  • Micrognathia or retrognathia
  • Low-set or posteriorly rotated ears
  • Abnormal forehead contour

1. Trisomy 21 (most common)

Facial pattern

  • Flat facial profile
  • Hypoplastic or absent nasal bone
  • Midface hypoplasia
  • Often mild micrognathia

Supporting clues

  • Short femur
  • EIF
  • Mild ventriculomegaly
  • AVSD or other cardiac defect

Trisomy 18

  • Micrognathia
  • Small, narrow face
  • Low-set ears
  • Often clenched hands, cardiac defects

Trisomy 13

  • Abnormal facial contour
  • Midline defects
  • Holoprosencephaly spectrum

Microdeletion / microduplication syndromes

Consider especially when:

  • Facies abnormal but NT not markedly increased
  • Structural anomalies present
  • Growth restriction

Key examples:

  • 22q11.2 deletion – Flat nasal bridge, small chin, conotruncal heart defects
  • Wolf–Hirschhorn (4p–) – "Greek helmet" facies, growth restriction
  • Smith–Lemli–Opitz – Facial anomalies + limb or genital findings

Ethnicity reduces the significance of isolated hypoplastic nasal bone, but does NOT reduce concern when facies are abnormal.

How risk changes

Scenario Risk interpretation
Isolated hypoplastic NB Low risk (ethnicity dependent)
Hypoplastic NB + subjective "flat face" Intermediate risk
Hypoplastic NB + objective facial anomalies High risk
Hypoplastic NB + facial + cardiac/CNS anomaly Very high risk