Led by Columbia University, the research was conducted through the Finnish Prenatal Study of Autism, based in a national birth cohort. The Finnish Hospital Discharge Register was used to identify children born in Finland between 1987 and 2005 and those diagnosed with an ASD by 2007. Overall, 2208 non-firstborn children with an ASD and 5163 matched controls were included. Conditional logistic regression, adjusted for any potential confounder, was used to determine any association between ASD and interpregnancy intervals (IPIs).
When compared with births that had an IPI of 24 to 59 months, those with an IPI of <12 months had an increased risk of ASD (odds ratio [OR] = 1.50, 95% confidence interval [CI] = 1.28, 1.74) on models adjusted for confounders. The ORs for longer IPIs, 60 to 119 months and ≥120 months, were 1.28 (95% CI = 1.08, 1.52) and 1.44 (95% CI = 1.12, 1.85), respectively.
Investigators concluded that there is evidence that longer and very short IPIs can increase the risk of ASD.