Background: Offspring of mothers with low levels of maternal vitamin D during pregnancy have been shown to have a range of adverse health outcomes, including the risk of autism spectrum disorder (ASD).Objectives: To determine the association between maternal serum 25(OH)-vitamin D concentrations during pregnancy, and behaviors consistent with ASD in offspring.Methods: Serum 25(OH)-vitamin D concentrations of 929 Caucasian women in Perth, Western Australia (32°S) were measured at 18 weeks pregnancy.Outcome data on the offspring were obtained in childhood & adolescence.Receptive language was assessed with Peabody Picture Vocabulary Test-Ⅲ & autistic-like traits measured with the Autism-Spectrum Quotient at 17 years (n =406).Multivariate regression analyses were used to examine associations.Results: Significant linear trends between quartiles of maternal vitamin D levels & language impairment at 5-(p<.05) & 10-years (p<.05) of age were shown.The risk of clinically-significant language difficulties was increased close to two-fold in children whose mothers had vitamin D insufficiency (-3d 46nmol/L) during pregnancy.No associations were found with adolescent autisticlike scores.Conclusions: Offspring language impairment is significantly associated with maternal vitamin D insufficiency during pregnancy.However, there was little evidence that maternal vitamin D levels during pregnancy are related to the ASD phenotype among offspring.