IntroductionThe Danish National Birth Cohort (DNBC) contains comprehensive information on diet, lifestyle, constitutional and other major characteristics of women during pregnancy. It provides a unique source for studies on health consequences of gestational diabetes mellitus. Our aim was to identify and validate the gestational diabetes mellitus cases in the cohort. Material and methodsWe extracted clinical information from hospital records for 1609 pregnancies included in the Danish National Birth Cohort with a diagnosis of diabetes during or before pregnancy registered in the Danish National Patient Register and/or from a Danish National Birth Cohort interview during pregnancy. We further validated the diagnosis of gestational diabetes mellitus in 2126 randomly selected pregnancies from the entire Danish National Birth Cohort. From the individual hospital records, an expert panel evaluated gestational diabetes mellitus status based on results from oral glucose tolerance tests, fasting blood glucose and Hb1c values, as well as diagnoses made by local obstetricians. ResultsThe audit categorized 783 pregnancies as gestational diabetes mellitus, corresponding to 0.89% of the 87 792 pregnancies for which a pregnancy interview for self-reported diabetes in pregnancy was available. From the randomly selected group the combined information from register and interviews could correctly identify 96% (95% CI 80-99.9%) of all cases in the entire Danish National Birth Cohort population. Positive predictive value, however, was only 59% (56-61%). ConclusionsThe combined use of data from register and interview provided a high sensitivity for gestational diabetes mellitus diagnosis. The low positive predictive value, however, suggests that systematic validation by hospital record review is essential not to underestimate the health consequences of gestational diabetes mellitus in future studies.