Prediagnostic Differences Narrowed Over Time in Danish Children Diagnosed With ADHD or ASD

As ADHD and autism spectrum disorder (ASD) diagnoses have increased, it remains unclear whether characteristics associated with diagnosis have changed. This population-based matched case-control study used Danish nationwide registries, including 71,317 individuals diagnosed before age 18 during 2012–2022 and 713,170 controls. Researchers assessed yearly associations between 19 prediagnostic characteristics and subsequent diagnosis using conditional logistic regression. Median age at diagnosis was 11.4 years; 37.1% of cases were female.

Cases differed from controls across all assessed characteristics, but associations generally weakened over time, particularly for socioeconomic and perinatal factors. The odds ratio for low birth weight decreased from 1.54 (95% CI, 1.41–1.68) in 2012–2013 to 1.17 (95% CI, 1.10–1.24) in 2020–2022. Attenuation was greater for ADHD than ASD and more pronounced among those diagnosed at ages 10–17 than at younger ages. Patterns were similar across sexes and in analyses restricted to cases with higher diagnostic certainty.

The findings suggest that increasing diagnostic rates may partly reflect changes in diagnostic practices or health care capacity, rather than changes in underlying risk alone. However, these observational associations do not establish the reasons for the trends. They also should not be interpreted as evidence that ADHD or ASD has become less impairing for individuals. Full text was unavailable for assessment.