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Adolescents appear to be at increased risk of mental illness during pregnancy and the first year postpartum, although most studies to date are small or limited to depression or self-harm. We aimed to compare the risks of psychiatric disorder diagnoses and adverse psychiatric outcomes during pregnancy and the first year postpartum between adolescents and adults.

In this retrospective, population-based cohort study, we used health administrative data from Ontario, Canada, to identify all singleton livebirth or stillbirths after 20 weeks of gestation between Jan 31, 2013, and March 31, 2021. We categorised births by the pregnant individual’s age at conception: adolescent (aged <20 years) or adult (aged ≥20 years). The main outcomes of interest were any psychiatric diagnosis between conception and 1 year postpartum, based on a physician’s diagnosis in an outpatient, emergency department, or inpatient setting, as well as adverse psychiatric outcomes (emergency department visit, inpatient admission, and self-injury). We used modified Poisson regression to compare adolescents and adults on the risk for outcome, generating relative risks (RRs) adjusted for sociodemographic characteristics, medical comorbidities, and parity. Young people with lived experience were involved in the study conduct and interpretation of findings.

The cohort comprised 992 729 births: 27 547 among adolescents and 965 182 among adults. Mean age at conception was 17·9 years (SD 1·2) for adolescents and 30·5 years (4·9) for adults. Adolescents were more likely to have a pre-pregnancy history of a psychiatric diagnosis than adults (12 625 [45·8%] vs 241 934 [25·1%]; standardised difference 0·44). A psychiatric disorder diagnosis in pregnancy or the first year postpartum was recorded in 10 973 (39·8%) adolescents and in 241 043 (25·0%) adults (adjusted RR 1·35 [95% CI 1·33–1·37]). 2485 (9·0%) adolescents and 15 013 (1·6%) adults visited the emergency department for a psychiatric reason during pregnancy or postpartum (adjusted RR 3·63 [95% CI 3·46–3·81]), 616 (2·2%) adolescents and 3795 (0·4%) adults had a psychiatric admission (3·67 [3·33–4·05]), and 320 (1·2%) adolescents and 1091 (0·1%) adults had an episode of self-injury (7·11 [6·12–8·26]).

Pregnant and postpartum adolescents are a distinct group at substantially increased risk of psychiatric diagnoses and adverse psychiatric outcomes. Targeted treatments to prevent and treat mental illness are urgently needed to improve outcomes in this population.