The etiology of inflammatory bowel disease (IBD) remains unspecified even though many genetic and non-genetic risk determinants are implicated. Emerging evidence indicates a prediagnostic phase of IBD which poses a window of opportunity for IBD prevention efforts.

The aims of the three studies comprised in this thesis were to explore and describe medication use in the prediagnostic phase of IBD; to investigate the impact of maternal IBD with prenatal vs. postnatal onset on offspring IBD risk; and to validate date of IBD diagnosis in the Danish National Patient Registry.

The first study showed that compared to a matched population, patients with IBD were more frequently users of almost every medication group already ten years before diagnosis. This supports the notion of a long prediagnostic phase of IBD and signals that this phase might include organ systems beyond the gastrointestinal tract. Our findings in the second study indicate that IBD risk is not ‘transferred’ to offspring through inflammatory factors in the intrauterine life but rather IBD risk in offspring seems to be a matter of having a first degree relative with IBD. What triggers IBD in offspring is still unknown. In the last study we found that the date of IBD diagnosis in the Danish National Patient Registry was valid which is reassuring for future studies investigating matters concerning the prediagnostic phase of IBD.

Continuing research in investigating the prediagnostic phase of IBD has the potential to aid early identification of IBD and ultimately to discover IBD etiology and causal mechanisms of disease development. This is