Anxiety disorders are the most common psychiatric disorders among children and adolescents. The aetiology of anxiety disorders is known to be multi-factorial and to include genetic and environmental factors. Prenatal and perinatal factors have been associated with other mental health disorders, but there has been less research on the role they play in anxiety disorders. The aim of this thesis was to comprehensively investigate the associations between prenatal and perinatal factors and child and adolescent anxiety disorders, as well as identifying the treated incidence of anxiety disorders.

The first step was to conduct a systematic literature review of the existing literature. The following studies were part of the Finnish Prenatal Study of Anxiety disorders (FIPS-Anx), which is an ongoing nested case-control study. This nationwide birth cohort included 22,388 cases, who were born in 1992 – 2006, and diagnosed with anxiety disorders in 1998 – 2012. Each identified case was matched with four controls. The data were obtained from three national Finnish registers: the Care Register for Health Care, the Finnish Medical Birth Register and the Finnish Population Register Centre.

Maternal low socioeconomic status and mother being single at the time of birth increased the odds for offspring anxiety disorders. Preterm birth and poor foetal growth increased the odds for anxiety disorders linearly but comorbid conditions of depressive and neurodevelopmental disorders explained these associations. Birth by caesarean section increased the odds for anxiety disorders. Differences in the associations for specific anxiety disorders were observed.

The thesis demonstrates associations between prenatal and perinatal factors and child and adolescent anxiety disorders. However, these associations were impacted by comorbid conditions and possibly some residual confounding factors. The findings support the complex multifactorial aetiology model for anxiety disorders rather than major findings for just prenatal or perinatal factors.