Autism Spectrum Disorder (ASD) is a serious neurodevelopmental disorder. Several previous studies have identified preterm birth as a risk factor for ASD but none has studied whether the association between gestational age and ASD has changed over time. This is a Danish population-based follow-up study including live-born singletons born in Denmark between 1980 and 2009, identified in the Danish Medical Birth Registry, a study population of 1,775,397 children. We used a Cox regression model combined with spline to study the risk for ASD by gestational age across three decades of birth cohorts. We included 19,020 children diagnosed with ASD. Across all birth year cohorts, we found that the risk of being diagnosed with ASD increased with lower gestational age (P-value: <0.01). Across all gestational weeks, we found a statistically significant higher risk estimates in birth cohort 1980 to 1989, compared to birth cohorts 1990 to 1999 and 2000 to 2009, respectively. No statistically significant difference in risk estimates was observed between birth cohort 1990 to 1999 and 2000 to 2009. The observed time trend in risk of ASD after preterm birth may reflect: (1) a change in the risk profile of persons with ASD due to the broadening of ASD diagnostic criteria over time; or (2) improved neonatal care for low GA infants, which has reduced risk of adverse outcomes like ASD in preterm children. Autism Res 2015. © 2015 International Society for Autism Research, Wiley Periodicals, Inc.