title: Nasal bone length, prenasal thickness, prenasal thickness-to-nasal bone length ratio and prefrontal space ratio in second- and third-trimester fetuses with Down syndrome.
abstract: OBJECTIVE
  To evaluate nasal bone length (NBL), prenasal thickness (PT), prenasal thickness-to-nasal bone length (PT-NBL) ratio and prefrontal space ratio (PFSR) as markers for Down syndrome in the second and third trimesters.
  
  
  METHODS
  NBL, PT, PT-NBL ratio and PFSR were measured retrospectively in stored two-dimensional images or three-dimensional volumes (corrected to the mid-sagittal plane) of fetuses with Down syndrome, which were retrieved from the digital databases of participating units. Measurements were performed on the stored images and volumes by two experienced operators, and the values obtained were compared to our previously reported normal ranges for euploid fetuses in order to assess the detection rates for Down syndrome.
  
  
  RESULTS
  A total of 159 fetuses with Down syndrome were included in the analysis, six of which were excluded because of inadequate available images. Median maternal age was 36.0 years and median gestational age was 23 + 1 weeks. NBL and PT were correlated with gestational age (P < 0.001), but the PT-NBL ratio and PFSR were not. Mean NBL, PT, PT-NBL ratio and PFSR were 4.42 mm, 5.56 mm, 1.26 and 0.34, respectively. The nasal bone was absent in 23 (15.4%) cases. As a marker for Down syndrome, the PT-NBL ratio yielded the highest detection rate (86.2%), followed by PFSR (79.7%), PT (63.4%) and NBL (61.9%). All markers were abnormal in 33.6% of cases, whilst all were normal in 4.7%. At least one of the four markers was abnormal in 95.3%, and either the PT-NBL ratio or PFSR was abnormal in 93.8%. Detection rates were not related to gestational age.
  
  
  CONCLUSIONS
  The PT-NBL ratio and PFSR are robust second- and third-trimester markers for Down syndrome. Both provide high detection rates and are easy to use, as the cut-off for normality is constant throughout gestation. Ltd.
authors.affiliation: Fetal Medicine Unit, Department of Obstetrics and Gynaecology, University Medical Centre Groningen, The Netherlands.
authors.forename: F I
authors.initials: FI
authors.lastname: Vos
authors.forename: E A P
authors.initials: EA
authors.lastname: De Jong-Pleij
authors.forename: M
authors.initials: M
authors.lastname: Bakker
authors.forename: E
authors.initials: E
authors.lastname: Tromp
authors.forename: E
authors.initials: E
authors.lastname: Pajkrt
authors.forename: K O
authors.initials: KO
authors.lastname: Kagan
authors.forename: C M
authors.initials: CM
authors.lastname: Bilardo
mesh_terms: D004314:Down Syndrome / Q000000981:diagnostic imaging*; D005145:Face / Q000002:abnormalities / Q000000981:diagnostic imaging*; D005260:Female; D005865:Gestational Age; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D009295:Nasal Bone / Q000002:abnormalities / Q000000981:diagnostic imaging*; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011263:Pregnancy Trimester, Third; D012189:Retrospective Studies; D016216:Ultrasonography, Prenatal / Q000379:methods
keywords: Down syndrome; nasal bone length; prefrontal space ratio; prenasal thickness; prenasal thickness-to-nasal bone length ratio; trisomy 21; ultrasound