Prenatal multivitamin/folic acid supplement use may reduce the risk of autism spectrum disorders. We investigated whether 2nd trimester prenatal vitamin use and maternal whole blood folate (WBF) concentrations were associated with Social Responsiveness Scale (SRS) scores at 4-5 years of age in a prospective cohort of 209 mother-child pairs. After confounder adjustment, children born to women taking prenatal vitamins weekly/daily (n = 179) had lower odds of clinically elevated SRS scores (odds ratio 0.26; 95 % confidence interval 0.08, 0.89) than those who rarely/never took them (n = 30). WBF concentrations were not associated with SRS scores. The lack of association between WBF and autistic-behaviors may be due to the timing of biomarker measures relative to critical periods of brain development, confounding, or other modifying factors.