23 years at 20 weeks 4 days. With nasal bone measured 4.8 mm
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nasal bone length normative values 20 weeks gestational age percentile chart 4.8mm
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Fetal Nasal Bone Assessment — 20 Weeks 4 Days
Is 4.8 mm Normal at 20+4 Weeks?
Yes — 4.8 mm is within the normal range at 20+4 weeks.
Key normative reference data for nasal bone length (NBL) at 20 weeks:
| Reference | 2.5th–5th Percentile at 20 weeks |
|---|---|
| Multiethnic Australian population (PMID 21806576) | 5.0 mm at the 2.5th centile |
| Indian population (PMID 24431652) | 3.6 mm at the 5th percentile |
| Philippines study | ≥4.50 mm considered normal at 20 weeks |
| Sonek 2003 (Ultrasound Obstet Gynecol) | Average NBL at 20 weeks ~6.0–6.5 mm |
Interpretation: At 20+4 weeks, 4.8 mm is borderline — it falls below the 2.5th centile in the Australian multiethnic population (which sets 5.0 mm at 20 weeks as the lower limit), but above the 5th percentile in South Asian references. This variability is clinically significant — ethnic background strongly affects interpretation.
Clinical Context: What Does a Short/Borderline Nasal Bone Mean?
Second-trimester nasal bone findings and Down syndrome risk:
- The fetal nasal bone is hypoplastic or absent in up to 60% of Down syndrome pregnancies imaged in the second trimester, but in only ~1–2% of unaffected pregnancies.
- Complete absence: ~37% of affected fetuses; Hypoplasia: ~50% of affected fetuses.
- In normal pregnancies: absence in 0.9%, hypoplasia in 2.4%.
NBL assessment is the best single ultrasound soft marker for second-trimester Down syndrome risk when performed by experienced operators, but must be interpreted in the context of the full clinical picture. — Creasy & Resnik's Maternal-Fetal Medicine, p. 655
Key Soft Marker Likelihood Ratios for Trisomy 21 (Isolated Findings)
| Marker | Likelihood Ratio |
|---|---|
| Nuchal thickening (>6 mm) | 17.0 (8.0–38) |
| Hyperechoic bowel | 6.1 (3.0–12.6) |
| Short/absent nasal bone | ~5–10 (varies by study) |
| Short humerus | 7.5 (4.7–12) |
| Echogenic intracardiac focus | 2.8 (1.5–5.5) |
| Short femur | 2.7 (1.2–6) |
| Pyelectasis | 1.5–1.9 |
Important Caveats
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Ethnicity matters significantly. Nasal bone absence/hypoplasia is more common in Afro-Caribbean (9%), South Asian (5%), and East Asian (3.3%) populations compared to Caucasians (2.2%) in euploid fetuses. NBL norms vary by population — use the correct reference range for the patient's ethnic background.
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Isolated finding vs. multiple markers. An isolated borderline nasal bone in a low-risk patient (age 23, no prior aneuploidy history) carries a very different risk profile than when combined with other soft markers.
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Soft markers should not independently calculate Down syndrome risk — they must be integrated into a total risk analysis incorporating maternal age, prior screening results (first trimester combined screen, cfDNA if done), and any other anatomic findings.
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Technical quality — NBL measurement requires midsagittal plane at ~45° angle. Suboptimal imaging can give false short measurements.
Recommended Next Steps
| Action | Rationale |
|---|---|
| Review complete anatomy survey | Look for additional soft markers or structural anomalies |
| Integrate with prior screening results | First-trimester NT/PAPP-A/β-hCG or cfDNA result |
| Consider referral to MFM/fetal medicine | For formal risk stratification |
| Offer cell-free DNA (cfDNA) if not done | High sensitivity/specificity for trisomy 21, non-invasive |
| Counsel regarding amniocentesis | If risk remains elevated after full assessment |
Bottom line: At 20+4 weeks, a nasal bone of 4.8 mm is borderline-short by some reference populations (notably below the 2.5th centile in multiethnic Australian data). In a 23-year-old with no other risk factors, this is a soft marker that modifies — but does not independently determine — Down syndrome risk. The clinical significance depends heavily on ethnicity, other scan findings, and prior screening results. MFM consultation and cfDNA testing (if not already performed) are appropriate next steps.
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