Determinants of number-specific recall error of last menstrual period: a retrospective cohort study

ObjectiveTo assess the digit preference for last menstrual period (LMP) dates, associated determinants and impact on obstetric outcome. DesignRetrospective cohort study. SettingUniversity medical centre (the Netherlands). PopulationCohort of 24665 LMP records and a subgroup of 4630 cases with known crown-rump length (CRL) measurement, and obstetric outcome. MethodsDigit 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. Main outcome measures(Non)deprived neighbourhood, cycle irregularity, certainty of LMP date, maternal age, smoking, body mass index, parity and ultrasound investigator. Preterm and post-term delivery. ResultsLMP 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 (42weeks) 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). ConclusionsLMP 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.