title: Intra- and interobserver variability in fetal ultrasound measurements.
abstract: OBJECTIVE
  To assess intra- and interobserver variability of fetal biometry measurements throughout pregnancy.
  
  
  METHODS
  A total of 175 scans (of 140 fetuses) were prospectively performed at 14-41 weeks of gestation ensuring an even distribution throughout gestation. From among three experienced sonographers, a pair of observers independently acquired a duplicate set of seven standard measurements for each fetus. Differences between and within observers were expressed in measurement units (mm), as a percentage of fetal dimensions and as gestational age-specific Z-scores. For all comparisons, Bland-Altman plots were used to quantify limits of agreement.
  
  
  RESULTS
  When using measurement units (mm) to express differences, both intra- and interobserver variability increased with gestational age. However, when measurement of variability took into account the increasing fetal size and was expressed as a percentage or Z-score, it remained constant throughout gestation. When expressed as a percentage or Z-score, the 95% limits of agreement for intraobserver difference for head circumference (HC) were ± 3.0% or 0.67; they were ± 5.3% or 0.90 and ± 6.6% or 0.94 for abdominal circumference (AC) and femur length (FL), respectively. The corresponding values for interobserver differences were ± 4.9% or 0.99 for HC, ± 8.8% or 1.35 for AC and ± 11.1% or 1.43 for FL.
  
  
  CONCLUSIONS
  Although intra- and interobserver variability increases with advancing gestation when expressed in millimeters, both are constant as a percentage of the fetal dimensions or when reported as a Z-score. Thus, measurement variability should be considered when interpreting fetal growth rates.
authors.affiliation: Oxford Maternal and Perinatal Health Institute, Green Templeton College and Nuffield Department of Obstetrics and Gynaecology, University of Oxford, John Radcliffe Hospital, Oxford, UK.
authors.forename: I
authors.initials: I
authors.lastname: Sarris
authors.forename: C
authors.initials: C
authors.lastname: Ioannou
authors.forename: P
authors.initials: P
authors.lastname: Chamberlain
authors.forename: E
authors.initials: E
authors.lastname: Ohuma
authors.forename: F
authors.initials: F
authors.lastname: Roseman
authors.forename: L
authors.initials: L
authors.lastname: Hoch
authors.forename: D G
authors.initials: DG
authors.lastname: Altman
authors.forename: A T
authors.initials: AT
authors.lastname: Papageorghiou
mesh_terms: D000328:Adult; D001699:Biometry; D005260:Female; D047109:Fetal Development; D005865:Gestational Age; D006801:Humans; D015588:Observer Variation; D011247:Pregnancy; D011446:Prospective Studies; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal / Q000379:methods