The Society for Maternal-Fetal Medicine (SMFM) states a hypoplastic nasal bone in the second trimester of pregnancy is defined as a ratio against the biparietal diameter (biparietal diameter-to-nasal bone ≥10 or ≥11), by length (≤2.5 mm), by gestational age-based percentiles (<2.5th percentile), or by multiples of the median (<0.75 or ≤0.7 MoM).
This calculator does not provide a definitive diagnosis “Hypoplastic”
depends on the reference standard used and overall risk assessment (age, screening, other markers, cfDNA, etc.).
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Disclaimer
This calculator computes Sonek percentile ranges (2.5th–97.5th) for 11–40 weeks, adds the Bunduki (Brazilian) population-specific percentiles for 16–24 weeks,
and summarizes Cusick second-trimester screening variables (MoM cutoffs, absent nasal bone, and BPD/NBL ratio) with an EMR-ready summary.
Definition mode controls how the tool labels “hypoplastic” for reporting.
Enter Gestational Age and Measurements
Leave blank or set as 0 if nasal bone is absent / not visualized, and check “Absent”.
Used for BPD/NBL ratio (variable).
The calculator still displays all references; this controls the “meets definition” label and EMR text.
Only applied to the Cusick MoM median table (15–23w).
Sonek 2.5/5/50/95/97.5Bunduki Brazil 2.5/5/50MoM + BPD/NBLEMR-ready
Technique reminder (quick)
Standard technique is a true midsagittal facial profile with clear visualization of the echogenic nasal bone.
Measurements vary with imaging plane and fetal position—interpret in clinical context.
Results
Ready
Metric
Value
Interpretation / Threshold checks
Enter GA (weeks + days) and click Calculate.
EMR-ready text
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References (in-page)
Society for Maternal-Fetal Medicine (SMFM). Electronic address: pubs@smfm.org; Prabhu M, Kuller JA, Biggio JR. Society for Maternal-Fetal Medicine Consult Series #57: Evaluation and management of isolated soft ultrasound markers for aneuploidy in the second trimester. Am J Obstet Gynecol. 2021 Oct;225(4):B2-B15. doi: 10.1016/j.ajog.2021.06.079. Epub 2021 Jun 23. PMID: 34171388.
Sonek JD, et al. Nasal bone length throughout gestation: normal ranges based on 3537 fetal ultrasound measurements. Ultrasound Obstet Gynecol. 2003;21(2):152-5.
PMID: 12601837
Bunduki V, et al. Fetal nasal bone length: reference range and clinical application in ultrasound screening for trisomy 21. Ultrasound Obstet Gynecol. 2003;21(2):156-60.
PMID: 12601838
Cusick W, et al. Likelihood ratios for fetal trisomy 21 based on nasal bone length in the second trimester: how best to define hypoplasia? Ultrasound Obstet Gynecol. 2007;30(3):271-4.
PMID: 17721915
