The risk of psychiatric disorders in individuals born prematurely in Denmark from 1974 to 1996

Aim: To investigate the risk of developing psychiatric disorders in individuals born prematurely (born before 37 weeks of gestation). Methods: The study is a longitudinal register‐based study including all live‐born individuals in Denmark from 1974 to 1996. Poisson regression models including well‐known confounders were used to investigate the rate ratio of psychiatric disorders at the age of 11–34 years in the individuals born prematurely. Results: A total of 1 545 641 individuals were born from 1974 to 1996. Of these, 1 348 106 live‐born individuals constituted the study cohort. Rate ratios of psychiatric disorders, in total, increased in proportion to the degree of prematurity. The rate ratio of major psychiatric disorder was increased in individuals born with a gestational age below 28 weeks compared to persons born with a gestational age of 39–45 weeks (RR = 1.88 (95% CI: 1.25–2.83)). Similarly, the rate ratios of minor psychiatric disorder (RR = 1.63 (95% CI: 1.29–2.06)) and psychotropic drugs purchased (RR = 1.35 (95% CI: 0.98–1.86)) were increased. Conclusion: The risk of minor as well as major psychiatric disorders during adolescence and young adulthood increases in proportion to the degree of prematurity. The relative risk, however, is small compared to the risks of cerebral palsy or low intelligence.