Differences in autism incidence among Norwegian-born preschool children by parental immigrant background and human development index in maternal country of birth

We assessed hazard of autism spectrum disorder among Norwegian-born preschool children with immigrant parents, categorized by human development index in maternal country of birth, relative to children with Norwegian-born parents. We further assessed the importance of household socioeconomic status and changes in hazard differences across birth cohorts. The sample included children 0–6 years born to two Norwegian-born or two immigrant parents between 2007 and 2017 (N = 527,863). A categorical measure of household SES, based on a combination of household income and parental education, was employed. Follow-up time was from 2009 or year of birth if later, until year of diagnosis, end of 2022, age of six, or year of death. Hazard ratios for an autism diagnosis by human development index groups were estimated using Cox regression, adjusting for birth year, sex, gestational age, and socioeconomic status, stratified by birth cohort, using children with Norwegian-born parents as the reference group. Children with immigrant parents had a higher hazard of autism, ranging from 2.28 (95% CI: 1.93–2.68) among those with mothers from very high human development index countries to 4.28 (3.61–5.06) among those with mothers from low human development index countries. Differences increased across birth cohorts: HR 1.62 (0.83–3.14) in 2007–2009 versus 6.96 (5.64–8.60) in 2014–2017. Findings suggest a widening gap in autism hazard between children with immigrant parents and those with Norwegian-born parents, especially for children with backgrounds from low human development index countries.