From some March of Dimes stuff I have...
"The key is having enough folic acid in your system by the early weeks of pregnancy, before the neural tube closes. Since many women don't discover that they're pregnant until this time, the March of Dimes, the Centers for Disease Control and Prevention, and the Institute of Medicine recommend that all women of childbearing age take a daily supplement with 400 micrograms of folic acid. The Institute of Medicine advises pregnant women and breast-feeding women to up their daily folic acid intake to 500 mcg and 600 mcg respectively, but these recommendations aren't intended to prevent neural tube defects. The increased amounts are needed because your body uses extra folic acid during pregnancy and lactation"