Three-dimensional ultrasound with maximal mode rendering: a novel technique for the diagnosis of bilateral or unilateral absence or hypoplasia of nasal bones in second-trimester screening for Down syndrome.

OBJECTIVE: Three-dimensional (3D) ultrasound of the fetal face with maximal mode rendering allows accurate visualization of the bony face and the distinct demonstration of both nasal bones in second-trimester fetuses. The aim of this study was to analyze the feasibility of assessing nasal bones spatially on prenatal ultrasound in second- and third-trimester fetuses with present and absent nasal bones. METHODS: The faces of 38 fetuses between 17 and 33 weeks' gestation were examined with 3D ultrasound and volumes were stored for offline evaluation. Eighteen fetuses had normal karyotype and an apparently normal nasal bone on 2D ultrasound; these were examined to standardize the 3D rendering technique. Twenty fetuses had trisomy 21. RESULTS: In all 18 healthy fetuses both nasal bones could be demonstrated on 3D ultrasound. Nine of the 20 Down syndrome fetuses had a hypoplastic or absent nasal bone on two-dimensional ultrasound. On 3D ultrasound three of the nine had discrepant findings between left and right nasal bones with evidence of absence of one side, and hypoplasia (n = 2) or normal length (n = 1) of the other. CONCLUSIONS: Unilateral absence or hypoplasia of nasal bones is an important and new observation in fetuses with Down syndrome. This differentiation is best demonstrated on 3D ultrasound with maximal mode rendering. This observation of unilaterality of findings could explain some discrepant findings on absence of nasal bones on two-dimensional ultrasound but their "presence" on lateral postmortem radiographs.