title: Prenasal thickness-to-nasal bone length ratio: a strong and simple second- and third-trimester marker for trisomy 21.
abstract: OBJECTIVE
  To study the ratio of prenasal thickness (PT) to nasal bone length (NBL) in normal and trisomy-21 fetuses in the second and third trimesters of pregnancy.
  
  
  METHODS
  The PT and NBL were measured retrospectively in 106 normal fetuses (in three-dimensional (3D) volumes) and in 30 fetuses with trisomy 21 (10 on two-dimensional (2D) images and 20 in 3D volumes).
  
  
  RESULTS
  In normal fetuses the mean PT and NBL increased between 15 and 33 weeks' gestation from 2.3 to 6.1 mm (r = 0.85, P < 0.001) and from 3.3 to 9.6 mm (r = 0.87, P < 0.001), respectively. The PT : NBL ratio was stable throughout gestation, with a mean of 0.61 (95% CI, 0.59-0.63; r = - 0.04, P = 0.7). The 5(th) and 95(th) percentiles were 0.48 and 0.80, respectively. In trisomy-21 fetuses the mean PT and NBL increased between 14 and 34 weeks from 3.0 to 9.2 mm (r = 0.86, P < 0.001) and from 1.9 to 7.8 mm (r = 0.85, P < 0.001), respectively. The PT : NBL ratio was significantly higher than in normal fetuses (P < 0.001) but also stable throughout gestation, with a mean of 1.50 (95% CI, 1.20-1.80; r = - 0.35, P = 0.07). Twenty-three (77%) of the 30 fetuses with trisomy 21 had a PT above the 95(th) percentile and 20 (67%) had an NBL below the 5(th) percentile. All the trisomy-21 fetuses had a PT : NBL ratio above the 95(th) percentile. When the 95(th) percentile of the PT : NBL ratio was used as a cut-off value the detection and false positive rates for trisomy 21 were 100 (95% CI, 89-100)% and 5 (95% CI, 2-11)%, respectively. The positive likelihood ratio was 21.2.
  
  
  CONCLUSIONS
  The PT : NBL ratio is stable in the second and third trimesters of pregnancy in both normal and trisomy-21 fetuses, but all trisomy-21 fetuses in this series had a PT : NBL ratio above the 95(th) percentile. The ratio is therefore a strong marker for trisomy 21.
authors.affiliation: Department of Obstetrics and Gynecology, St Antonius Hospital, Nieuwegein, Utrecht, The Netherlands. e.de.jong@antoniusziekenhuis.nl
authors.forename: E A P
authors.initials: EA
authors.lastname: De Jong-Pleij
authors.forename: F I
authors.initials: FI
authors.lastname: Vos
authors.forename: L S M
authors.initials: LS
authors.lastname: Ribbert
authors.forename: L R
authors.initials: LR
authors.lastname: Pistorius
authors.forename: E
authors.initials: E
authors.lastname: Tromp
authors.forename: C M
authors.initials: CM
authors.lastname: Bilardo
mesh_terms: D000328:Adult; D004314:Down Syndrome / Q000175:diagnosis* / Q000196:embryology / Q000473:pathology; D005260:Female; D006801:Humans; D007621:Karyotyping; D008423:Maternal Age; D008875:Middle Aged; D009295:Nasal Bone / Q000002:abnormalities* / Q000000981:diagnostic imaging*; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011263:Pregnancy Trimester, Third; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal