Ketil Størdal

Aim To examine possible geographical and temporal differences in the incidence of childhood‐onset inflammatory bowel disease (IBD) in Norway, motivated by previous research indicating relevant environmental factors explaining changing epidemiology. Methods We analysed data from children born in Norway from 2004 to 2012 ( n = 541 036) in a registry‐based nationwide study. After validating registry diagnoses against medical records, we defined IBD as ≥2 entries of International Classification of Diseases, 10th revision (ICD‐10) codes K50, K51 and K52.3 in the Norwegian Patient registry. We estimated hazard ratios (HR) for IBD across four geographical regions with a south‐to‐north gradient and the incidence by period of birth. Results By the end of follow‐up on 31 December 2020, 799 IBD diagnoses were identified (Crohn's disease: n = 465; ulcerative colitis, n = 293, IBD: unclassified, n = 41). Compared to children in the southernmost region, there was almost a two‐fold HR for IBD in children in the most Northern region (HR = 1.94, 95% Cl = 1.47–2.57; Mid region: HR = 1.68, 95% CI = 1.29–2.19, p trend <0.001). These estimates remained largely unchanged after adjustment for potential confounding factors. The cohorts born in 2004–2006 and 2010–2012 had comparable cumulative incidences, with a slightly higher incidence for those born in 2007–2009. Conclusion We observed an increase in the risk of IBD by increasing latitude which may suggest that environmental factors influence the development of IBD, although non‐causal explanations cannot be ruled out.