Ossified Nasal Bone at 22 Weeks: Normal Finding
An ossified (present and normally formed) nasal bone at 22 weeks gestation is a completely normal and reassuring finding that effectively excludes nasal bone hypoplasia as a soft marker for trisomy 21. 1
Understanding Nasal Bone Assessment at 22 Weeks
The nasal bone should be routinely visualized during the second-trimester anatomic survey, which is typically performed between 18-22 weeks gestation. 2 At 22 weeks, the presence of an ossified nasal bone is the expected normal finding in the vast majority of pregnancies.
Normal Nasal Bone Characteristics
- Normal length at 22 weeks: The mean nasal bone length at 22 weeks is approximately 6.8 mm, with normal values ranging above the 2.5th percentile for gestational age. 3
- Measurement technique: The nasal bone must be imaged perpendicular to the longitudinal axis of the nose in the midsagittal plane of the fetal face for accurate assessment. 1
- Prevalence of normal ossification: In chromosomally normal fetuses, an ossified nasal bone is present in 98-99% of cases at this gestational age. 3, 4
Clinical Significance of Present Nasal Bone
The presence of a normally ossified nasal bone at 22 weeks provides strong reassurance against trisomy 21. The likelihood ratio for trisomy 21 when the nasal bone is present is only 0.38 (95% CI 0.24-0.56), meaning it substantially reduces the risk. 4
What Constitutes Abnormal (Hypoplastic) Nasal Bone
Hypoplastic nasal bone in the second trimester is defined by multiple criteria: 1
- Biparietal diameter-to-nasal bone ratio ≥10 or 11
- Absolute length <2.5 mm
- Measurement below the 2.5th percentile for gestational age
- Multiples of the median <0.75 or 0.7 MoM
Since your finding describes an ossified (present) nasal bone, none of these abnormal criteria apply.
Management Implications
No Further Action Required
- No additional antenatal ultrasound surveillance is recommended for a normally ossified nasal bone. 1
- No aneuploidy screening or diagnostic testing is indicated based on this finding alone. 1
- The presence of a normal nasal bone at 22 weeks is simply documented as part of the routine anatomic survey. 2
Context of Prior Screening
If the patient has already undergone aneuploidy screening:
- With negative cell-free DNA screening: A present nasal bone provides additional reassurance, and no further evaluation is needed. 1
- With negative serum screening: The normal nasal bone finding supports the low-risk status. 1
- Without prior screening: The presence of a normal nasal bone is reassuring but does not replace standard aneuploidy screening protocols if clinically indicated. 1
Important Clinical Caveats
Ethnic Variation in Nasal Bone Assessment
While a present nasal bone is reassuring across all populations, it's worth noting that nasal bone hypoplasia (when absent) shows significant ethnic variation, occurring in 0.5% of Caucasians but up to 8.8% of Afro-Caribbeans in chromosomally normal fetuses. 4 However, this variation is irrelevant when the nasal bone is present and normally ossified, as in your case.
Timing of Assessment
- First trimester (11-14 weeks): Nasal bone assessment is more commonly used as a screening tool, with absence of the nasal bone being a stronger marker for trisomy 21 at this earlier gestational age. 2, 5, 6
- Second trimester (19-22 weeks): Nasal bone hypoplasia remains a valid soft marker, but the presence of a normal nasal bone at this stage is the expected finding. 2, 3
Soft Markers in Context
The ACR guidelines emphasize that soft markers like nasal bone assessment have diminished relevance in the era of first-trimester screening and cell-free DNA testing. 2 The presence of a normal nasal bone at 22 weeks simply confirms normal fetal development and requires no special consideration or follow-up. 1