What is the incidence and clinical significance of congenitally absent nasal bone?

Incidence of Congenitally Absent Nasal Bone

The incidence of congenitally absent nasal bone in euploid (chromosomally normal) pregnancies ranges from 0.1% to 1.2%, with significant ethnic variation—occurring in approximately 2.8% of Caucasians, 6.8% of Asians, and up to 9-10.4% of Afro-Caribbean populations. 1, 2

Population-Specific Incidence Data

Overall Euploid Population

• In unselected chromosomally normal pregnancies, absent nasal bone occurs in 0.1% to 1.2% of cases 1
• Large prospective studies demonstrate rates of approximately 0.2% to 2.8% in euploid fetuses when ethnic variation is considered 3, 2

Ethnic Variation (Critical for Interpretation)

• Caucasian populations: 2.8% incidence 2
• Asian populations: 6.8% incidence 2
• Afro-Caribbean populations: 9% to 10.4% incidence 1, 2
• The American College of Obstetricians and Gynecologists emphasizes that ethnic background must not be overlooked when interpreting this finding, as normal variant rates differ substantially 1

Gestational Age Considerations

The incidence varies by crown-rump length (CRL) at first-trimester screening 2:

• CRL 45-54 mm: 4.6% absent nasal bone
• CRL 55-64 mm: 3.9% absent nasal bone
• CRL 65-74 mm: 1.5% absent nasal bone
• CRL 75-84 mm: 1.0% absent nasal bone

Clinical Significance in Aneuploid Fetuses

Association with Trisomy 21

• Absent or hypoplastic nasal bone demonstrates a positive likelihood ratio of 23 when combined with other markers, but only 6.6 when isolated 1
• In fetuses with trisomy 21, the nasal bone is absent in 66.9% to 70% of cases 3, 2
• The prevalence of Down syndrome in fetuses with absent nasal bone is approximately 8.5% compared to 0.42% in fetuses with normal nasal bone present 4

Association with Other Aneuploidies

• Trisomy 18: absent in 57.1% to 80% of cases 3, 2
• Trisomy 13: absent in 31.8% of cases 2
• Turner syndrome: absent in 8.8% to 66% of cases 3, 2

Relationship to Nuchal Translucency

The incidence of absent nasal bone increases with nuchal translucency (NT) thickness in euploid fetuses 2:

• NT < 2.5 mm: 1.8% absent nasal bone
• NT 2.5-3.4 mm: 3.4% absent nasal bone
• NT 3.5-4.4 mm: 5.0% absent nasal bone
• NT ≥ 4.5 mm: 11.8% absent nasal bone

When NT measures ≥3 mm, additional first-trimester markers such as absent nasal bone may be useful for counseling, though the increased NT itself warrants further evaluation regardless 5

Outcomes in Euploid Fetuses

Isolated Finding

• In euploid fetuses with isolated absent nasal bone and otherwise normal second-trimester anatomy, normal newborn examination findings provide reassurance 6
• The American Institute of Ultrasound in Medicine states that no additional antenatal ultrasound surveillance is recommended for isolated hypoplastic nasal bone alone 1

Associated Anomalies

• All euploid cases with adverse outcomes in one series had additional prenatal sonographic findings beyond the absent nasal bone 6
• Absent nasal bone can be associated with other genetic syndromes beyond common aneuploidies, including microdeletion syndromes and specific genetic diagnoses 1, 6
• Careful evaluation of fetal anatomy is warranted, as chromosomal microarray may be more sensitive than standard karyotype in detecting chromosomal aberrations 4

Common Pitfalls to Avoid

• Ignoring ethnic background: Failure to account for the patient's ethnicity can lead to unnecessary anxiety and testing, as rates vary from 2.8% to 10.4% in normal populations 1, 2
• Recommending invasive testing after negative cell-free DNA: The American College of Obstetricians and Gynecologists advises against invasive testing solely for isolated hypoplastic nasal bone after negative cell-free DNA screening, as this finding is most likely a normal variant in the presence of low aneuploidy risk 1
• Inadequate imaging technique: The nasal bone must be imaged perpendicular to the longitudinal axis of the nose in the midsagittal plane for accurate assessment 1