AbstractAimWe examined the associations between preterm birth, poor foetal growth and anxiety disorders among children and adolescents. Additionally, we examined the impact of common comorbidities and specific anxiety disorders separately.MethodsThree Finnish registers provided data on a nationwide birth cohort of 22,181 cases with anxiety disorders and 74,726 controls. Conditional logistic regression was used to examine the associations.ResultsExtremely very preterm birth and moderate‐late preterm birth were associated with increased adjusted odds ratios (aOR) for anxiety disorders (aOR 1.39, 95% CI 1.11–1.75 and aOR 1.13, 95% CI 1.03–1.23, respectively). Weight for gestational age of less than −2SD (aOR 1.29, 95% CI 1.17–1.42) and −2SD to −1SD (aOR 1.08, 95% CI 1.03–1.14) were associated with increased odds ratios for anxiety disorders. When comorbidities were considered, the associations became statistically insignificant for pure anxiety disorders, but remained significant in the groups with comorbid depressive or neurodevelopmental disorders.ConclusionPreterm birth and poor foetal growth increased the odds for anxiety disorders. However, the associations seem to be explained by the conditions of comorbid depressive and neurodevelopmental disorders. Comorbidities should be considered when examining and treating child and adolescent anxiety disorders.