Between 2006 and 2010, two research-validated instruments, Social Communication Questionnaire (SCQ) and Social Responsiveness Scale (SRS) were filled out online by 4,188 mothers of Autism Spectrum Disorder (ASD) children, aged 4-21, as part of voluntary parental participation in a large web-based registry. Univariate and multivariate linear regression analysis (adjusted for child's sex, ability to verbalize, categorical IQ score, and fetal growth rate) demonstrated significantly higher SCQ and SRS scores for ASD children of both preterm (less than 37 weeks) and post-term (greater than 42 weeks) gestational age (GA) compared to ASD children of normal GA, thus indicating that both preterm and post-term children manifest increased ASD symptomatology. Normal GA at birth appears to mitigate the severity of autistic social impairment in ASD children. (Contains 7 tables and 1 figure.)