Taking Prenatal Supplements Associated with 30% Lower Risk of Autism

Correlation or causation? I would think parents who care about vitamins and prenatal care do generally pay more attention to their own wellbeing and preparation for the child, which I expect to have positive outcomes for the child, both prenatal and a potential sign for better postnatal care. How much do the supplements themselves matter? In such studies, only rarely can they be done in a proper scientific way (double blind etc).