Nasal Bone (NB) Measuring Method During

Step-by-Step Nasal Bone Measurement Method


Nasal Bone (NB) Measuring Method During First NT screening
  • NB is assessed between 11 weeks 0 days and 13 weeks 6 days (CRL: 45–84 mm).
  • The midsagittal plane of the fetal face should be used – same as for NT measurement.
  • The fetal head must be in a neutral position, not flexed or extended.
  • The ultrasound image should be magnified so that the fetal head and upper thorax fill the screen.
  • The nasal bone appears as a bright echogenic line under the skin line of the fetal nose.
  • The NB is assessed as either present, absent, or hypoplastic (underdeveloped).
  • The insonation angle should be 45° to the nasal bridge to avoid false absence.
  • Calipers are placed on the outer edges of the echogenic nasal bone line (if measured).
  • Multiple images should be taken and the best one used for documentation.

Nasal Bone (NB) Assessment Criteria

Plane: True mid-sagittal view of the fetal profile (same as NT view)
Fetal: Position head in neutral position, no flexion or hyperextension
Zoom: Fetal head and upper thorax should occupy at least 75% of the image
Magnification: Maximize to clearly visualize nasal bridge and skin
Image Quality: High-resolution grayscale with appropriate gain (to avoid under/overexposing bones)

Landmarks for NB

  • Skin line (top)
  • Nasal bone (middle, shorter and more echogenic)
  • Tip of the nose (hard palate or nasal tip) (bottom)
CRL vs Nasal Bone Length
CRL vs Nasal Bone Length Reference
CRL (mm) Approx. Gestational Age (weeks+days) Expected NB Length (mean ± SD) Range (approx.)
45–54 mm ~11w0d to 11w6d 1.4 ± 0.2 mm 1.1 – 1.8 mm
55–64 mm ~12w0d to 12w6d 1.7 ± 0.2 mm 1.3 – 2.1 mm
65–74 mm ~13w0d to 13w6d 1.9 ± 0.2 mm 1.5 – 2.3 mm
75–84 mm ~14w0d to 14w6d 2.1 ± 0.2 mm 1.7 – 2.5 mm

short or absent nasal bone

1. Trisomy 21 (Down Syndrome)
Absent nasal bone is seen in up to 60–70% of fetuses with Down syndrome.
Short nasal bone is also commonly associated.
Particularly significant when seen between 11–14 weeks (first trimester).

2. Other Chromosomal Abnormalities
May also be seen (less commonly) in:
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Turner syndrome


NB Length Percentile Calculator

NB Length Percentile Calculator

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