Research on Autism spectrum disorder (ASD) trends and risk factors, particularly relating to by intellectual disability (ID), is limited. This study examined the incidence, sociodemographic risk factors and co-occurrence conditions of ASD, including categorization by ID, using national registers.

This study included singletons born in Finland between 1998 and 2015 who had been diagnosed with ASD by 2018, with cases categorized into ASD with ID and ASD without ID. We divided the study sample into four birth cohorts (1998–2002, 2003–2007, 2008–2011 and 2012–2015) to analyze changes in incidence over time. Cases (n = 10,171) were matched with controls (n = 49,391) by age, gender, and birthplace. Associations between sociodemographic risk factors and ASD were analyzed using conditional logistic regression. Co-occurrence with other psychiatric disorders was examined only in the oldest cohort (1992–2002).

The cumulative incidence of ASD without ID increased from 0.52 to 0.89% by age 10, while ASD with ID remained stable at 0.17%. Several socio-demographic risk factors were associated with both groups, while parental immigration status was only associated with cases with ID. A total of 59.0% of cases had one co-occurring psychiatric disorder, with a significant difference in prevalence between the groups (p < .05).

The increase in diagnosed ASD, particularly without ID, recorded by specialized services in Finland between 1998 and 2018 may reflect a real increase in incidence, or changes in diagnostic criteria and practices, improved mental health services, greater public and professional awareness or treatment seeking behavior.