Development of the defect in the neural tube occurs within the first month after conception, before most women are aware of their pregnancy. Because supplements containing folic acid at the 0.4-mg level are widely available, this dosage has been the focus of the available observational research studies. It is possible that lower doses of folic acid may reduce the risk for NTDs, but further research would be needed to learn the minimum effective dose. Over the years, RDAs have ranged from 0.18 to 0.4 mg of folic acid for women of childbearing age. RDAs for pregnant women have ranged from 0.4 to 0.8 mg per day.
Folic acid is a water-soluble vitamin, and any excess consumed is rapidly excreted in the urine.
Therefore, women should be careful to keep their total daily folate consumption at less than 1 mg per day. Caution should also be taken to prevent excessive use of multivitamin supplements or fortified foods containing vitamin A, since excess vitamin A may cause birth defects. Further research will be needed to identify any unknown adverse effects. It is expected that consumption of adequate folic acid will avert some, but not all, NTDs. The underlying causes of NTDs are not known.