The present invention provides diagnostic methods for determining the risk of developing an autism spectrum disorder (ASD) in a fetus or child by detecting in a biological sample from the mother antibodies that bind to one or more biomarkers selected from the group consisting of lactate dehydrogenase (LDH), guanine deaminase (GDA), collapsin response mediator protein 1 (CRMP1), stress-induced phosphoprotein 1 (STIP1), alpha subunit of the barbed-end actin binding protein Cap Z (CAPZA2), Y Box Binding Protein 1 (YBX1), eukaryotic translation and elongation factor 1A1 (EEF1A1), microtubule-associated protein Tau (MAPT), dihydropyrimidinase-like protein 2 (DPYSL2), dynamin 1-like protein (DNM1L), radixin (RDX), moesin (MSN), and ezrin (EZR). The invention further provides methods of preventing or reducing the risk of a fetus or child developing an ASD by administering to the mother an agent that blocks the binding of maternal antibodies to the one or more fetal biomarkers listed above or by removing from the mother antibodies that bind to the one or more fetal biomarkers.