**4. The nasal bone and nostrils**

A small nose is very commonly seen during postnatal examination of fetuses or neonates who also present trisomy 21 as well as for more than 40 other genetic problems. The nasal bone can be measured using a mid-sagittal profile for normal singleton fetuses between the 14th and 34th week of gestation. Thus, the length of the nasal bones increase from 4 mm at 14 weeks to 12 mm at 35 weeks gestation. A possible improvement in screening for trisomy 21 by examining the fetal nasal bone with ultrasound at 11–14 weeks of gestation has been considered [22].
