Prevention of Neural Tube Defects
In 1992 the U.S. Public Health Service recommended that all women of childbearing age consume 400 micrograms (0.4 mg) of folic acid daily. The Centers for Disease Control and Prevention (CDC) estimate that if all women of childbearing age take folic acid, a B vitamin, daily before and during early pregnancy, the incidence of birth defects of the brain and spine (called neural tube defects or NTDs) could be reduced by up to 70 percent.

Because 50 percent of all pregnancies in the United States are unplanned or incorrectly timed and because neural tube defects occur during the first month of pregnancy, before most women know they are pregnant, all women of childbearing age should consume 400 micrograms of folic acid daily.

Prevention of the Recurrence of an NTD-Affected Pregnancy
In 1991 the U.S. Public Health Service (USPHS) recommended that all women who have already had an NTD-affected pregnancy should consume 4 milligrams (4000 micrograms) of folic acid, through prescription, daily. They should begin this dosage one month before they start trying to get pregnant and continue through the first three months of pregnancy. These women should follow the USPHS guidelines even when not planning to become pregnant.

Source: Centers for Disease Control. Recommendations for the use of folic acid to reduce the number of cases of spina bifida and other neural tube defects. Morbidity and Mortality Weekly Report, volume 41, number RR-14, September 11, 1992.