Investigating Implausible Gestational Age and High Birthweight Combinations.

Background: Birth certificate data overestimate national preterm births because a high percentage of last menstrual period (LMP) dates have errors. Study goals were to determine: (i) To what extent errors in transfer of birthweight and LMP date from medical records to birth certificates contribute to implausibly high birthweight-for-gestational-age births; (ii) What percentage of implausible births would be resolved if the clinical estimate (CE) from birth certificates were used instead of LMP-based gestational age, and with what degree of certainty; and (iii) Of those not resolved, what percentage had a medical explanation.

Methods: Medical records and birth certificates for all singleton infants with implausibly high birthweight-for-gestational-age based on LMP delivered in the Kaiser Permanente Northwest system in Oregon during 1998-2007 were examined. Percentages of implausible records resolved under various scenarios were calculated.

Results: A total of 100 births with implausibly high birthweight-for-gestational age combinations were identified. When LMP date and birthweight from medical records were used instead of from birth certificates, 31% of births with implausible combinations were resolved. Substituting the CE on the birth certificate for the LMP date resolved 92%. Of the latter, the clinician's gestational age estimate in the medical record was obtained in early pregnancy in 72%. Five of the eight births with unresolved implausible combinations were to mothers with diabetes; the remaining three had no documented medical explanation.

Conclusions: In this study, use of the birth certificate CE rather than the LMP resulted in a clinically reliable reclassification for the majority of implausible birthweight-for-gestational age deliveries.