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.
Bibliography
Gynaecologia 