Ultrasonographic measurement of fetal nasal bone in a low-risk population at 19-22 gestational weeks.

OBJECTIVE: To determine the potential value of sonographic measurement of fetal nasal bone at 19-22 weeks' gestation in screening for trisomy 21 in a low-risk population. METHODS: The fetal nasal bone was measured in a mid-sagittal view in 2035 fetuses at 19-22 weeks' gestation. A reference range was constructed and the measurements in fetuses with trisomy 21 were compared to the normal group. RESULTS: The fetal profile was successfully examined in 1913/2035 (94%) fetuses. The mean nasal bone length increased linearly with gestation from 6.2 mm at 19 weeks to 6.8 mm at 22 weeks. Nasal bone hypoplasia, defined by absence of the bone or a measurement below the 2.5th centile, was observed in 34/1899 (1.8%) chromosomally normal fetuses (1.8%), in 5/5 fetuses with trisomy 21 and in 0/9 fetuses with other chromosomal defects. CONCLUSION: At 19-22 weeks' gestation, nasal bone hypoplasia is observed in a high proportion of trisomy 21 fetuses and in less than 2% of chromosomally normal fetuses.