To examine risks of attention-deficit/hyperactivity disorder (ADHD) in preterm and early term birth survivors, and potential sex-specific differences.
A national cohort study was conducted of all 4,061,795 singletons born in Sweden in 1973-2013 who survived infancy, followed up for ADHD identified from nationwide diagnoses and medications through 2018. Poisson regression was used to compute prevalence ratios (PRs), adjusting for sociodemographic and perinatal factors. Co-sibling analyses assessed for confounding by unmeasured shared familial (genetic or environmental) factors.
ADHD prevalences by gestational age at birth were 12.1% for extremely preterm (22-27 weeks), 7.0% for moderately preterm (28-33 weeks), 5.7% for late preterm (34-36 weeks), 6.1% for all preterm (<37 weeks), 5.2% for early term (37-38 weeks), and 4.5% for full-term (39-41 weeks). Adjusted PRs comparing extremely preterm, all preterm, or early term vs. full-term, respectively, were 2.35 (95% CI, 2.15-2.57), 1.28 (1.25-1.31), and 1.12 (1.10-1.13) among males, and 2.46 (2.17-2.78), 1.24 (1.20-1.28), and 1.08 (1.06-1.10) among females (P<0.001 for each). These associations were virtually unchanged after controlling for shared familial factors. Both spontaneous and medically indicated preterm birth were associated with ADHD (adjusted PRs, 1.21; 95% CI, 1.18-1.24; and 1.39; 1.34-1.43, respectively).
In this large cohort, preterm and early term birth were associated with increased risks of ADHD in males and females, independently of covariates and shared familial factors.

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