Absent nasal bone

The ultrasound specialist should be aware that there is a high association between absent nasal bone at 11–13+6 weeks and trisomy 21, as well as other chromosomal abnormalities. The nasal bone is absent in 60–70% of trisomy 21 fetuses, in about 50% of trisomy 18 fetuses and 30% of trisomy 13 fetuses. One should not forget that in chromosomally normal fetuses the incidence of absent nasal bone is less than 1% in Caucasian populations and about 10% in Afro-Caribbeans. In the second trimester nasal hypoplasia (absent nasal bone or smaller than 2.5 mm) is likely to be the single most sensitive and specific marker of trisomy 21.

Fetal nasal bone
A chromosomally normal fetus with a normal nasal bone. Note the hyperechogenic tip of the nose, the hyperechogenic skin of the nose above the nasal bone and the nasal bone itself.
Absent fetal nasal bone
Absent nasal bone in a fetus with trisomy 21.

Bibliography

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