Objective The aim of the present study was to investigate whether premature birth is associated with psychiatric disease at adult age.

Methods The study was a longitudinal register based study including two cohorts of individuals born in Denmark in the 2 periods: 1974 to 1976 and 1980 to 1982. The two cohorts were divided by their gestational age in three groups: very preterm individuals born before 33 weeks of gestation (VPT), preterm individuals born at 33 to 36 weeks of gestation (PT), and term individuals born after 36 weeks of gestation (term). Psychiatric disease was expressed by 1) the cumulated incidence of a psychiatric diagnosis since 1978, and 2) by the use of neurotropic medication in the last 11 years.

Results The cumulated incidence of psychiatric diagnoses was higher in the young adults born preterm compared to those born at term (OR 1.3 (p<0.001)/OR = 1.6 (p<0.001)). The differences between the two preterm groups were only significant in the 1980–82 cohort. These results were corroborated by the prescriptions of neurotropic drugs in the two cohorts.

Conclusions The association between preterm birth and later psychiatric disease in adulthood was weak. The odds ratios were modest, but the risk differences point to a significant problem, perhaps explained by comorbidity associated with neurological disease.