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

To assess the digit preference for last menstrual period (LMP) dates, associated determinants and impact on obstetric outcome.

Retrospective cohort study.

University medical centre (the Netherlands).

Cohort of 24 665 LMP records and a subgroup of 4630 cases with known crown-rump length (CRL) measurement, and obstetric outcome.

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.

(Non)deprived neighbourhood, cycle irregularity, certainty of LMP date, maternal age, smoking, body mass index, parity and ultrasound investigator. Preterm and post-term delivery.

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).

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.