Reliability of variables on the North Carolina birth certificate: a comparison with directly queried values from a cohort study.

Birth records are an important source of data for examining population-level birth outcomes, but questions about the reliability of these vital records exist. We sought to assess the reliability of birth certificate data by comparing them with data from a large prospective cohort. Pregnancy, Infection, and Nutrition cohort study participants were matched with their birth certificates to assess agreement for maternal demographics, health behaviours, previous pregnancies and major pregnancy events. Agreement among categorical variables was assessed using percentage agreement and kappa statistics; for continuous variables, Spearman's correlations and concordance correlation coefficients were used. The majority of variables had high agreement between the two data sources, especially for maternal demographic and birth outcome variables. Variables measuring anaemia, gestational diabetes and alcohol consumption showed the lowest correlations. Number of cigarettes smoked and number of previous pregnancies differed by education categories. For most variables, birth records appear to be a good source of reliable information. With the exception of a few variables that differed by education, most variables did not differ by stratum of race or education. Our research further supports the use of birth certificates as a reliable source of population-level data.