Results of adulthood mental health of those born late-preterm (34 + 0–36 + 6 weeks + days of gestation) are mixed and based on national registers. We examined if late-preterm birth was associated with a higher risk for common mental disorders in young adulthood when using a diagnostic interview, and if this risk decreased as gestational age increased.

A total of 800 young adults (mean = 25.3, s.d. = 0.62 years), born 1985–1986, participated in a follow-up of the Arvo Ylppö Longitudinal Study. Common mental disorders (mood, anxiety and substance use disorders) during the past 12 months were defined using the Composite International Diagnostic Interview (Munich version). Gestational age was extracted from hospital birth records and categorized into early-preterm (<34 + 0, n = 37), late-preterm (34 + 0–36 + 6, n = 106), term (37 + 0–41 + 6, n = 617) and post-term (⩾42 + 0, n = 40).

Those born late-preterm and at term were at a similar risk for any common mental disorder [odds ratio (OR) 1.11, 95% confidence interval (CI) 0.67–1.84], for mood (OR 1.11, 95% CI 0.54–2.25), anxiety (OR 1.00, 95% CI 0.40–2.50) and substance use (OR 1.31, 95% CI 0.74–2.32) disorders, and co-morbidity of these disorders (p = 0.38). While the mental disorder risk decreased significantly as gestational age increased, the trend was driven by a higher risk in those born early-preterm.

Using a cohort born during the advanced neonatal and early childhood care, we found that not all individuals born preterm are at risk for common mental disorders in young adulthood – those born late-preterm are not, while those born early-preterm are at a higher risk. Available resources for prevention and intervention should be targeted towards the preterm group born the earliest.