In a high proportion of fetuses with trisomy 21 and other chromosomal abnormalities the nasal bone is hypoplastic or not visible at 11-13 weeks' gestation. Assessment of the nasal bone at 11-13 weeks improves the performance of combined screening for trisomy 21 by maternal age, fetal nuchal translucency (NT) and serum biochemistry. The difficulty is when the gestation is 11 weeks or the beginning of the 12th week and the nasal bone is absent but the NT, the other ultrasound markers and the serum biochemistry are normal. Our advice is that in such cases the scan should be repeated in one week and if there is persistence of absence of the nasal bone then the risk for trisomies is increased.
