• birth records;
  • LMP date;
  • accuracy;
  • gestational age


Birth certificate last menstrual period (LMP) date is widely used to estimate gestational age in the US. While data quality concerns have been raised, no large population-based study has isolated data quality issues by comparing birth record LMP (Birth LMP) with reliable LMP dates from another source. We assessed LMP data quality in 2002 California singleton livebirth records (n = 515 381) and in a subset of records with linked prenatally collected LMP from California's statewide Prenatal Expanded Alpha-fetoprotein Screening Program (XAFP) (n = 105 936). Missing or incomplete LMP data affected 13% of birth records; 17% of those had complete LMP within XAFP records.

Data quality indicators supported XAFP LMP as more accurate than Birth LMP, with a lower prevalence of digit preference, post-term delivery, out-of-range gestational age estimates and implausible birthweight-for-gestational age. The bimodal birthweight distribution evident at 20–31 weeks' gestation based on Birth LMP was nearly absent with XAFP LMP-based gestational age. Approximately 32% of the second birthweight mode was explained by apparent clerical errors in Birth LMP month. Digit preference errors, particularly day 1, were associated with gestational age overestimation. Preterm delivery rates were higher according to Birth (7.6%) vs. XAFP LMP (7.2%). One-fifth of observed preterm and over half of observed post-term births using Birth LMP were not true cases; 15% of true preterm cases were missed. African American or Hispanic, less educated, and publicly or uninsured women were most likely to be misclassified and have large LMP date discrepancies attributable to clerical or digit preference error. The implementation of a revised birth certificate is an opportunity for targeted training and data entry checks that could substantially improve LMP accuracy on birth records.