Nasal bone

In the assessment of the fetal nasal bone the gestation should be 11⁺⁰ to 13⁺⁶ weeks and the CRL 45-84 mm.

• The magnification of the image should be such that the head and upper thorax occupy the whole screen.
• A mid-sagittal view of the fetal profile should be obtained.
• The ultrasound transducer should be parallel to the direction of the nose and the probe must be gently tilted from one side to the other of the fetal nose.
• When these criteria are satisfied, three distinct lines should be seen at the level of the fetal nose:
• The top line represents the skin.
• The bottom one, which is thicker and more echogenic than the overlying skin, represents the nasal bone.
• A third line in front of the bone and at a higher level than the skin represents the tip of the nose.
• The nasal bone is considered to be present if it is more echogenic than the overlying skin and absent if it is either not visible or its echogenicity is the same or less than that of the skin.

Rotation of the head by about 10° away from the midline results in non-visibility of the tip of the nose and the appearance of the maxillary bone as an echogenic structure between the nasal bone above and the anterior part of the palate below. With further rotation from the midline the nasal bone disappears and there is enlargement of the maxillary bone and coalescence with the palate.