Trends and incidence rates of neurodevelopmental disorders in Danish children and adolescents 2000–2024: Short title: neurodevelopmental disorders in children and adolescents

Background: Autism Spectrum Disorder (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are prevalent neurodevelopmental disorders with long-term consequences. Recognition is essential but often delayed, particularly in females. Many cases are not captured in national registers, though many ADHD are often identifiable through prescriptions. We examined age- and sex-specific incidence and temporal trends in ASD and ADHD diagnosis in Denmark from 2000 to 2024 and assessed how including prescription data affects ADHD estimates. Methods: A cohort of all children residing in Denmark from 2000 to 2024 (n = 2.8 million) was followed until age 18 or December 31, 2024. Using nationwide registers, we identified diagnoses of ASD and ADHD. For ADHD, we extended the definition to include ADHD-specific prescriptions. We estimated age- and sex-specific incidence rates (IRs), cumulative incidence at each 1-year age point, and trends by birth cohort. Results: Among 2.8 million children, 60,432 were diagnosed with ASD and 102,051 with ADHD. Incidence rates increased for both conditions, with a steeper rise among females. By age 18, cumulative incidence was 6.5% (95% confidence intervals (CI) 6.5–6.6) in males and 3.9% (95% CI 3.8-4.0) in females for ASD, and 9.8% (95% CI 9.8–9.9) and 7.4% (95% CI 7.3–7.5) for ADHD. Although rates remained higher in males, the sex gap narrowed substantially, particularly for ADHD. Including ADHD-specific prescriptions raised cumulative ADHD incidence by age 18 from 6.9% (95% CI 6.8–6.9) to 8.7% (95% CI 8.6–8.7). Conclusion: The incidence of ASD and ADHD has risen substantially in Denmark over the past two decades, most pronounced among girls, narrowing the historical sex gap. Incorporating prescription data increased ADHD incidence estimates notably, underscoring the importance of broader case definitions in epidemiological research.