Gestational and genetic factors can contribute to autism during infancy and early childhood through their effects on fetal brain development. Previous twin studies have shown strong genetic components for the development of autism, a disorder that can have multiple causes. We investigated the effects of prenatal overstimulation of the beta2-adrenergic receptor in dizygotic twins who were exposed to terbutaline, a selective beta2-adrenergic receptor agonist used to treat premature labor, as a gestational factor. As a possible genetic mechanism, we studied two beta2-adrenergic receptor polymorphisms in twins from whom DNA was available: glycine substitution at codon 16 (16G) and glutamic acid substitution at codon 27 (27E), which show diminished desensitization in vivo compared with the wild-type receptor. Continuous terbutaline exposure for 2 weeks or longer was associated with increased concordance for autism spectrum disorders in dizygotic twins (relative risk = 2.0), with a further increase in the risk for male twins with no other affected siblings (relative risk = 4.4). A significant association was found between the presence of 16G and 27E polymorphisms in autistic patients compared with population controls (P = .006). Prenatal overstimulation of the beta2-adrenergic receptor by terbutaline or by increased signaling of genetic polymorphisms of the beta2-adrenergic receptor that have diminished desensitization can affect cellular responses and developmental programs in the fetal brain, leading to autism.