title: Determinants of number-specific recall error of last menstrual period: a retrospective cohort study.
abstract: OBJECTIVE
  To assess the digit preference for last menstrual period (LMP) dates, associated determinants and impact on obstetric outcome.
  
  
  METHODS
  Retrospective cohort study.
  
  
  METHODS
  University medical centre (the Netherlands).
  
  
  METHODS
  Cohort of 24 665 LMP records and a subgroup of 4630 cases with known crown-rump length (CRL) measurement, and obstetric outcome.
  
  
  METHODS
  Digit preference was determined by comparing the observed to expected counts of each day. Associated determinants were identified by multivariate regression analysis. Differences in obstetric outcome between LMP and CRL dating were analysed.
  
  
  METHODS
  (Non)deprived neighbourhood, cycle irregularity, certainty of LMP date, maternal age, smoking, body mass index, parity and ultrasound investigator. Preterm and post-term delivery.
  
  
  RESULTS
  LMP digit preference for the first [odds ratio (OR), 1.28; 95% confidence interval (95% CI), 1.20-1.36], fifth (OR, 1.10; 95% CI, 1.03-1.17), 10th (OR, 1.17; 95% CI, 1.09-1.25), 15th (OR, 1.31; 95% CI, 1.23-1.40), 20th (OR, 1.22; 95% CI, 1.15-1.30) and 25th (OR, 1.08; 95% CI, 1.01-1.15) days of the month occurred more often than expected. Digit preference occurred more frequently in women living in a deprived neighbourhood (OR, 1.21; 95% CI, 1.06-1.39), with uncertain LMP (OR, 2.03; 95% CI, 1.63-2.52) or irregular cycle (OR, 1.24; 95% CI, 1.06-1.44). More post-term (≥42 weeks) deliveries (OR, 1.27; 95% CI, 1.05-1.54) were observed in LMP dating. This effect was larger in women with a digit preference (OR, 1.56; 95% CI, 1.03-2.37).
  
  
  CONCLUSIONS
  LMP digit preference occurs more often in women living in deprived neighbourhoods, with uncertain LMP or an irregular cycle. LMP-dated pregnancies are associated with more post-term pregnancies.
authors.affiliation: Division of Obstetrics and Prenatal Medicine, Department of Obstetrics and Gynaecology, Erasmus MC, University Medical Centre, Rotterdam, The Netherlands.
authors.forename: Rhf
authors.initials: R
authors.lastname: van Oppenraaij
authors.affiliation: Department of Biostatistics, Erasmus MC, University Medical Centre, Rotterdam, the Netherlands.
authors.forename: Phc
authors.initials: P
authors.lastname: Eilers
authors.affiliation: Department of Biostatistics, Erasmus MC, University Medical Centre, Rotterdam, the Netherlands.
authors.forename: S P
authors.initials: SP
authors.lastname: Willemsen
authors.affiliation: Department of Obstetrics and Gynecology, Medical Centre Haaglanden, The Hague, the Netherlands.
authors.forename: F M
authors.initials: FM
authors.lastname: van Dunné
authors.affiliation: Division of Obstetrics and Prenatal Medicine, Department of Obstetrics and Gynaecology, Erasmus MC, University Medical Centre, Rotterdam, The Netherlands.
authors.forename: N
authors.initials: N
authors.lastname: Exalto
authors.affiliation: Division of Obstetrics and Prenatal Medicine, Department of Obstetrics and Gynaecology, Erasmus MC, University Medical Centre, Rotterdam, The Netherlands.
authors.forename: Eap
authors.initials: E
authors.lastname: Steegers
mesh_terms: D015982:Bias; D015331:Cohort Studies; D018568:Crown-Rump Length; D005260:Female; D005865:Gestational Age; D006801:Humans; D016015:Logistic Models; D008598:Menstruation; D011939:Mental Recall; D015999:Multivariate Analysis; D016017:Odds Ratio; D011247:Pregnancy; D011273:Pregnancy, Prolonged / Q000175:diagnosis*; D012189:Retrospective Studies; D047929:Term Birth; D013997:Time Factors; D016216:Ultrasonography, Prenatal
keywords: Digit preference; menstrual period; pregnancy dating