The absence or marked hypoplasia of nasal bone in early gestation can help to identify a fetus with aneuploidy. This finding can be combined with other sonographic and biochemical markers to reach a fairly accurate diagnosis.
Gestational age: lower limit 11 weeks; upper limit 25 weeks
Recommended steps for sonographic evaluation of the fetal nasal bone:
(1) The study should only be done by someone trained and experienced in the technique.
(2) The fetus should be in the mid-sagittal plane with the spine down.
(3) Orient the ultrasound beam so that the angle of insonation is 45°.
(4) Magnify the image until the region of the fetal nose is identified.
(5) Tilt the transducer from side to side in order to distinguish fetal skin from nasal bone.
(6) The nasal bone is seen as a triangular echogenic structure.
If nasal bone is detected then its length is measured.
Aneuploidy is suspected when the nasal bone is absent or hypoplastic (length < 2.5th percentile for fetuses of that gestational age).
Absence of the nasal bone at a gestational age of 11-14 weeks had a sensitivity of 40-80% and specificity of 97-100% for trisomy 21.
The biparietal diameter to nasal bone ratio can be used to identify nasal bone hypoplasia.
biparietal diameter to nasal bone ratio =
= (biparietal diameter in mm) / (nasal bone length in mm)
A ratio >= 11 at a gestational age of 11 to 22 weeks was associated with aneuploidy by Odibo et al with a sensitivity of 50% and specificity of 93%.
Limitations:
• It may be impossible to visualize the region of the nasal bone in some fetuses.
