"Take 400 micrograms of folic acid daily both before pregnancy and during the first few months of pregnancy to reduce the risk of birth defects of the brain and spine. All women who could possibly become pregnant should take a vitamin with folic acid, every day. It is also important to eat a healthy diet with fortified foods (enriched grain products, including cereals, rice, breads, and pastas) and foods with natural sources of folate (orange juice, green leafy vegetables, beans, peanuts, broccoli, asparagus, peas, and lentils).
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http://www.cdc.gov/ncbddd/bd/abc.htm