title: Normal ranges for fetal nasal bone length determined by ultrasound at 18-20 weeks of gestation in a multiethnic Australian population.
abstract: OBJECTIVE
  Absence or hypoplasia of the nasal bone is commonly reported in Down syndrome fetuses. We define normal ranges and the 2.5th centile for fetal nasal bone length (NBL) in a multiethnic Australian population at 18-20 weeks of gestation.
  
  
  METHODS
  A prospective cohort study of women attending for a routine anomaly scan. Ethnicity of the patient and their partner was recorded, and the nasal bone was measured three times. Two methods of nasal bone assessment were used to define normal ranges: a single (first) measurement and the mean value of three measurements. Mixed-effects regression models were employed to account for interoperator differences treating sonographers as random effects. Nonparametric methods were used to define the 2.5th centile for gestational age.
  
  
  RESULTS
  A total of 1199 women were included with a mean gestational age 19.1 (SD 0.4; range 18-20) weeks. There is significant linear relationship between NBL and gestational age (P<0.001). The mean of three nasal bone measurements had a smaller standard deviation than single nasal bone measurements. Nonparametric assessment was used to define the 2.5th centile, which is 4.4 mm at 18 weeks and 5.0 mm at 20 weeks of gestation.
  
  
  CONCLUSIONS
  This study provides a reference range for fetal NBL at 18-20 weeks of gestation in an unselected multiethnic Australian population. Whilst NBL increases linearly from 18 to 20 weeks, the data are not normally distributed and nonparametric techniques are required to define the 2.5th centile. The mixed-effects model also accounts for variation in sonographer measurements.
authors.affiliation: Department of Obstetric and Gynaecological Ultrasound, Royal Prince Alfred Hospital, and Faculty of Obstetrics and Gynaecology, Central Clinical School, University of Sydney, Sydney, Australia. ritumogra@hotmail.com
authors.forename: Ritu
authors.initials: R
authors.lastname: Mogra
authors.forename: Philip
authors.initials: P
authors.lastname: Schluter
authors.forename: Robert
authors.initials: R
authors.lastname: Ogle
authors.forename: Mary
authors.initials: M
authors.lastname: Walter
authors.forename: Michael
authors.initials: M
authors.lastname: Borg
authors.forename: Jon
authors.initials: J
authors.lastname: Hyett
mesh_terms: D000293:Adolescent; D000328:Adult; D001315:Australia / Q000208:ethnology; D015331:Cohort Studies; D003430:Cross-Sectional Studies; D005006:Ethnicity; D005260:Female; D005865:Gestational Age; D006801:Humans; D008875:Middle Aged; D009295:Nasal Bone / Q000000981:diagnostic imaging / Q000196:embryology*; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011446:Prospective Studies; D012016:Reference Values; D016216:Ultrasonography, Prenatal; D055815:Young Adult