Folic acid supplements at the recommended intake of 400 micrograms per day about 3 months before and during pregnancy can help prevent neural tube defects. However, high doses of folic acid supplements of 1000 micrograms per day or more can negatively impact the cognition of the child.
Desirée Valera-Gran, Eva M Navarrete-Muñoz, Manuela Garcia de la Hera, et al. Effect of maternal high dosages of folic acid supplements on neurocognitive development in children at 4–5 y of age: the prospective birth cohort Infancia y Medio Ambiente (INMA) study. First published July 19, 2017, doi: 10.3945/ajcn.117.152769 Am J Clin Nutr September 2017 vol. 106 no. 3 878-887 http://ajcn.nutrition.org/content/106/3/878.abstract?etoc