title: Reproducibility study of crown-rump length and biparietal diameter measurements in the first trimester.
abstract: OBJECTIVE
  This study aims to investigate the intra-observer and inter-observer variability of crown-rump length (CRL) and biparietal diameter (BPD) measurements in the first trimester.
  
  
  METHODS
  A prospective observational study on 592 fetuses between 8 and 14 gestational weeks was conducted. Intra-class correlation coefficients (ICCs) were computed, and Bland-Altman analysis was carried out.
  
  
  RESULTS
  The ICCs (95% confidence interval) and the 95% limits of agreement, expressed as a percentage of the average, for CRL's intra-observer and inter-observer variability were 0.979 (0.974-0.982, p < 0.001), +8.53% to -8.376%, and 0.968 (0.952-0.978, p < 0.001), +9.06% to -7.69%, respectively. The corresponding figures for BPD were 0.981 (0.977-0.984, p < 0.001), +7.41% to -7.51%, and 0.968 (0.952-0.978, p < 0.001), +6.65% to -7.25%. After conversion in days, the ICCs and 95% limits of agreement for CRL's intra-observer and inter-observer variability were 0.983 (0.980-0.986, p < 0.001), +2.88% to -2.84%, and 0.971 (0.957-0.980, p < 0.001), 2.83% to -2.60%, respectively. The corresponding figures for BPD were 0.982 (0.979-0.986, p < 0.001), +3.36% to -3.40%, and 0.968 (0.953-0.978, p < 0.001), +3.06% to -3.38%. CRL's standard deviation of the differences increased with gestation for intra-observer (r = 0.289, p < 0.001) and inter-observer (r = 0.197, p = 0.023) variability.
  
  
  CONCLUSIONS
  The BPD and CRL are highly reproducible measurements. CRL's measurement error increased with the magnitude of CRL, whereas BPD's reproducibility was not affected by gestational age.
authors.affiliation: Fetal Medicine Unit, 3rd Department of Obstetrics and Gynaecology, University of Athens, Attikon' University Hospital, Athens, Greece. athena.souka@googlemail.com
authors.forename: Athena P
authors.initials: AP
authors.lastname: Souka
authors.forename: Athanasios
authors.initials: A
authors.lastname: Pilalis
authors.forename: Ioannis
authors.initials: I
authors.lastname: Papastefanou
authors.forename: George
authors.initials: G
authors.lastname: Salamalekis
authors.forename: Dimitrios
authors.initials: D
authors.lastname: Kassanos
mesh_terms: D002508:Cephalometry / Q000379:methods / Q000592:standards* / Q000706:statistics & numerical data; D018568:Crown-Rump Length; D005260:Female; D006801:Humans; D015588:Observer Variation; D011247:Pregnancy; D011261:Pregnancy Trimester, First / Q000502:physiology; D012016:Reference Values; D015203:Reproducibility of Results; D016216:Ultrasonography, Prenatal / Q000379:methods / Q000592:standards* / Q000706:statistics & numerical data