Background Autism spectrum disorders (ASDs) are disorders of neural development characterised by impaired social interaction and communication, and by restricted and repetitive behaviour. Only few previous studies have investigated neonatal conditions and the risk for ASDs.

Objectives To use Danish population based sample and register based information to investigate whether neonatal conditions are associated to the later development of ASD.

Methods A Danish population based cohort study, including all singletons born in Denmark from 1994, through 2005, a total of 581 493 children. Data were retrieved from the Danish National Hospital Register and the Danish Psychiatric Central Register. Data were analysed using Cox proportional hazards regression. All analyses were stratified by gestational age (term vs preterm birth).

Results A total of 4846 children were diagnosed with ASD during the follow-up time. We found an increased risk of ASD after exposure to a variety of neonatal conditions. For children born at term, we found an increased risk of ASD after perinatal hypoxia: HR 5.0 (95% CI 2.1 to 11.9), neonatal seizures: 2.2 (1.4 to 3.5), intracranial haemorrhage: HR 3.0 (1.4 to 6.2), neonatal hypoglycemia: HR 1.5 (1.3 to 1.8), and neonatal septicaemia or meningitis: HR 1.8 (1.5 to 2.2). The results for children born preterm were similar as for children born at term.

Conclusions Different neonatal conditions are likely to cause neurological damage and alter brain development, and hence increase the risk of ASDs. This effect seems to be mediated through different pathways including lack of oxygen, glucose, and possibly through activated immune function during early neonatal life.