Introduction Recently, the autism age-specific prevalence has increased dramatically and autism prevalence estimates now range from 60 to 70/10 000. This makes autism the second most common neurodevelopmental disorder after mental retardation (MR) and a serious public health concern.
Autism is a highly heritable multifactorial disorder, but the specific etiological pathways are largely unknown. Previous studies reported ORs from 1.9 to 2.3 for autism in low birth weight children (lbw, <2500 g) when compared to normal birth weight children (nbw, 3000–3999 g), but most studies have ignored the possible impact of comorbidities. Co-existing developmental disabilities (CDDs) like MR, ADHD, and epilepsy are common in autism.
We hypothesise that lbw is a stronger risk factor for autism with CDDs than for autism without CDDs. We report HR (95% CI) for autism + CDDs and autism / noCDDs specifically in lbw children when compared to nbw children.
Methods We conducted a nationwide, register-based, follow-up study. 1990 to 2007 birth cohorts were identified in The Danish Medical Birth Register and followed through 2009. Linkage with national hospital registers made exposure, outcome, and covariates information available to us.
Results We found HR 2.0 (1.5 to 2.8) for autism + MR and HR 0.9 (0.7 to 1.1) for autism/no MR in lbw children when compared to nbw children. Analyses of more CDD subgroups are ongoing.
Conclusion Consistent with a recently published study we find that lbw children have a twofold increased risk for autism with MR but no increased risk for autism with normal intelligence. In etiological studies, subgrouping autism cases on the basis of CDDs may enhance our knowledge of etiological pathways in autism.