A randomized, controlled trial found that pregnant women who took a specific probiotic supplement had zero cases of ADHD or Asperger syndrome in their children. Here is a link to the full study: A possible link between early probiotic intervention and the risk of neuropsychiatric disorders later in childhood: a randomized trial
The pregnant women in the study took Lactobacillus rhamnosus GG (ATCC 53103) for one month prior to delivery and for the first 6 months while breastfeeding, and none of children in the probiotic intervention group went on to receive an ADHD or Asperger diagnosis within the first 13 years of their life. Meanwhile, 17% of the children in the placebo group (who did not receive the probiotic) were diagnosed with either ADHD or Asperger syndrome by 13 years of age.
Even though this was a small preliminary study of only 75 participants, the results were nonetheless highly statistically significant (P = .008). Probiotic supplements (especially those that are well researched) are generally regarded as safe. Given the results of this study and the relative safety of well-studied probiotic supplements, a recommendation for pregnant women and nursing mothers to take a specific probiotic supplement (Lactobacillus rhamnosus GG) or eat fermented foods containing probiotics, would be a simple health intervention that could dramatically reduce rates of ADHD and Asperger syndrome in children. Furthermore, probiotics early in life could potentially provide other microbiome-related benefits to health and well-being.