title: Lower accuracy in prediction of delivery date in Stockholm County following introduction of new guidelines.
abstract: BACKGROUND
  In Stockholm County, new guidelines for predicting the day of delivery were introduced in 2010. Recent clinical use has indicated that the predictive quality might be suboptimal. This study compares the accuracy of three equations applied during the first (crown-rump length or bi-parietal diameter) and second trimesters (bi-parietal diameter or bi-parietal diameter combined with femur length).
  
  
  METHODS
  The accuracy of estimated delivery date was compared in 14 239 ultrasound exams using median deviations from actual birth date, proportion of postterms, births within ±seven days of estimated delivery date, accuracy variations depending on fetal size, alternative fetal measurements obtained on the same occasion, and menstrual age.
  
  
  RESULTS
  The bi-parietal diameter and crown-rump length formulae overestimated pregnancy length by two and three days respectively, causing 7-8% of pregnancies to be labelled postterm. A combined bi-parietal diameter+femur length formula overestimated by one day, with 5.1% postterms. No significant difference was found in the proportion of births within ±seven days. Second trimester estimated delivery date assessment had larger median variations than did first trimester assessment and suffered from shifting deviations across fetal size. The comparison of different biometry formulae in the same individual demonstrated one day extra deviation for bi-parietal diameter and three days extra deviation for crown-rump length compared with the combined bi-parietal diameter+femur length formula.
  
  
  CONCLUSIONS
  The algorithms and dating occasions tested seem inappropriate for the present 280-day term definition. Alternative formulae ought to be sought, and the assumed duration of pregnancy reconsidered; 283 days corresponds to the observed pregnancy length calculated from last menstrual period, and would better fit the observed results for first trimester ultrasound scans.
authors.affiliation: Department of Women's and Children's Health, Karolinska Institute, Stockholm, Sweden.|Ultragyn i Stockholm AB, Stockholm, Sweden.
authors.forename: Hans
authors.identifier: http://orcid.org/0000-0002-6399-807X
authors.initials: H
authors.lastname: Wessel
authors.affiliation: Department of Clinical Cancer Epidemiology, Karolinska Institute, Stockholm, Sweden.
authors.forename: Tommy
authors.identifier: http://orcid.org/0000-0002-9436-0626
authors.initials: T
authors.lastname: Nyberg
mesh_terms: D000328:Adult; D000465:Algorithms; D001699:Biometry / Q000379:methods*; D002508:Cephalometry; D018568:Crown-Rump Length; D036861:Delivery, Obstetric; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging; D005865:Gestational Age; D006801:Humans; D017410:Practice Guidelines as Topic; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011262:Pregnancy Trimester, Second; D016216:Ultrasonography, Prenatal
keywords: Gestational age; bi-parietal diameter; crown-rump length; estimated date of delivery; femur length; fetal biometry; postterm pregnancy; quality assessment