Isolated absent or hypoplastic nasal bone in the second trimester fetus: is amniocentesis necessary?

To elucidate the significance of isolated absent or hypoplastic nasal bone in the second trimester ultrasound scan.

All cases of absent or hypoplastic nasal bone (length < 5th percentile) encountered during 2007-2009 were retrieved from database and all the ultrasound findings including structural abnormalities and soft markers for Down syndrome and fetal karyotype were reviewed. The cases were categorized into a study group with isolated absent or hypoplastic nasal bone and a comparison group with additional ultrasound findings. The incidence of Down syndrome confirmed by karyotyping was compared between the two groups.

Among 14 fetuses with absent or hypoplastic nasal bone identified, six (42.9%) had Down syndrome and eight (57.1%) were normal. All (100%) of the six fetuses with isolated absent or hypoplastic nasal bone (Study Group) had normal karyotype, while six (75%) of the other eight fetuses with additional ultrasound findings (Comparison Group) had Down syndrome (p = 0.010).

The use of isolated absent or hypoplastic nasal bone in the second trimester ultrasound scan for Down syndrome screening may not be effective. Amniocentesis, however, is indicated for fetuses with structural abnormality or additional soft marker which should be carefully searched by an experienced ultrasonographer.