This study evaluated if elemental concentrations differed in children with an autism spectrum disorder (ASD) versus normally developing children, at the pre-natal level of development, by means of a sibling-paired design, using deciduous teeth as biomarkers of pre-natal exposure to the elements. A total of 22 families were sampled from the Southern Ontario region (London, Hamilton, Orangeville, Newmarket, Toronto, Kingston and Ottawa) in which the full set of deciduous teeth from all children, where only one child was diagnosed with an ASD, were acquired. An analogous sample was collected from 7 control families. The concentrations of K, Mn, Na, Pb and Sr in the deciduous teeth of children with an ASD were found not to differ from that of their normally developing siblings and the control group (α = 0.01). The concentrations of Mg, Ni, Cu, Fe and Zn were found to be lower in the deciduous teeth of children with an ASD versus their normally developing siblings, while Cr concentrations were found to be elevated (ρ < 0.001). The differences were correlated to a general trend by which the concentrations decreased (or increase, in the case of Cr) in children as the mother conceived more children within the family. This may indicate that the trends and differences observed may be a secondary effect to another underlying condition, presumably at the level of the mother.