title: The use and accuracy of manual and electronic gestational age calculators.
abstract: OBJECTIVE
  The accuracy of gestational age (GA) wheels has been shown to be poor, yet they remain commonly used. We surveyed their use within our institution and determined the inter-observer and inter-device variability of a range of devices. We have devised a procedure for validating device accuracy.
  
  
  METHODS
  All clinicians within our maternity unit were asked their most recent method of calculating GA and whether they felt this was accurate to within one day. Ten clinicians assessed 16 devices: 14 manual, 2 electronic. Five dates represented the last menstrual period (LMP) and were used to calculate the estimated date of delivery (EDD) of the 5 dates for each device compared to a 280 day control. ANOVA was used for statistical analysis.
  
  
  RESULTS
  73% last used a manual device to calculate GA. Seventy-two per cent believed their method was accurate. There was a significant bias (difference in the mean) between device-calculated and control EDD for all manual devices except one, with individual differences of up to 4 days. Variability altered throughout the year. Electronic devices consistently had no error. Inter-observer variability was insignificant.
  
  
  CONCLUSIONS
  Clinicians should be aware of the inaccuracy of manual devices. Electronic devices are recommended. Manual devices should be validated before use by comparing the device-calculated EDD with a 280 day control at five points throughout the year.
authors.affiliation: Department of Reproduction and Endocrinology, Maternal and Fetal Research Unit, London, UK. Elizabeth.smout@kcl.ac.uk
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Smout
authors.forename: Paul T
authors.initials: PT
authors.lastname: Seed
authors.forename: Andrew H
authors.initials: AH
authors.lastname: Shennan
mesh_terms: D000704:Analysis of Variance; D036861:Delivery, Obstetric; D005260:Female; D005865:Gestational Age; D006801:Humans; D008432:Mathematical Computing; D055641:Mathematical Concepts; D008597:Menstrual Cycle; D015588:Observer Variation; D011247:Pregnancy; D015203:Reproducibility of Results; D012621:Seasons; D011795:Surveys and Questionnaires; D013997:Time Factors