title: Use of Z-scores to select a fetal biometric reference curve.
abstract: OBJECTIVE
  Fetal biometric data are a major part of prenatal ultrasound screening in the general population. The aim of this study was to analyze the effect of choice of reference curve on the quality of screening for growth abnormalities, using a statistical tool based on Z-scores.
  
  
  METHODS
  The biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL) were measured in 9699 ultrasound scans during the second trimester (20-24 weeks of gestation) and 8100 scans during the third trimester (30-34 weeks of gestation). These biometric data were all transformed retrospectively into Z-scores, calculated using five reference curves: those published by Snijders and Nicolaides (1994), Chitty et al. (1994), Kurmanavicius et al. (1999) and Salomon et al. (2006), and curves used at our ultrasound unit generated from a sample of the local population. The Z-score distribution was compared with the expected normal distribution by calculation of the mean and SD, and using the Kolmogorov-Smirnov test. The sensitivity and specificity of each reference curve were calculated to determine the capacity of these curves to identify fetuses with measurements < 5(th) percentile or > 95(th) percentile for each parameter.
  
  
  RESULTS
  Most of the distribution curves determined from the Z-scores of the measurements taken differed significantly from a non-skewed standard normal curve (mean of 0 and SD of 1). In our population, the Chitty reference curves gave the best results for identifying fetuses with abnormal (< 5(th) percentile or > 95(th) percentile) BPD (sensitivity, 100%; specificity, 97.24%), HC (sensitivity, 96.07%; specificity, 98.89%) and FL (sensitivity, 96.46%; specificity, 98.80%). The best reference for AC was the Salomon curve (sensitivity, 72.25%; specificity, 99.64%).
  
  
  CONCLUSIONS
  Checking for good concordance between the study population and chosen reference data is a key initial step in quality control. Z-scores are a simple tool for evaluating the performance of each reference curve for a given population in order to optimize the sensitivity and specificity of screening for fetal growth abnormalities.
authors.affiliation: Department of Ultrasound and Fetal Medicine, Centre Médico-Chirurgical et Obstétrical-Syndicat Inter-Hospitalier de la Communauté Urbaine de Strasbourg, Strasbourg, France.
authors.forename: N
authors.initials: N
authors.lastname: Sananes
authors.forename: V
authors.initials: V
authors.lastname: Guigue
authors.forename: M
authors.initials: M
authors.lastname: Kohler
authors.forename: N
authors.initials: N
authors.lastname: Bouffet
authors.forename: M
authors.initials: M
authors.lastname: Cancellier
authors.forename: F
authors.initials: F
authors.lastname: Hornecker
authors.forename: M C
authors.initials: MC
authors.lastname: Hunsinger
authors.forename: A
authors.initials: A
authors.lastname: Kohler
authors.forename: C
authors.initials: C
authors.lastname: Mager
authors.forename: M
authors.initials: M
authors.lastname: Neumann
authors.forename: E
authors.initials: E
authors.lastname: Schmerber
authors.forename: M
authors.initials: M
authors.lastname: Tanghe
authors.forename: I
authors.initials: I
authors.lastname: Nisand
authors.forename: R
authors.initials: R
authors.lastname: Favre
mesh_terms: D001699:Biometry / Q000379:methods*; D005260:Female; D005317:Fetal Growth Retardation / Q000000981:diagnostic imaging*; D005865:Gestational Age; D006801:Humans; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011263:Pregnancy Trimester, Third; D011785:Quality Assurance, Health Care; D012016:Reference Values; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal / Q000592:standards*