Spina bifida and anencephaly are common and serious birth defects. Available evidence indicates that 0.4 mg (400 micrograms) per day of folic acid, one of the B vitamins, will reduce the number of cases of neural tube defects (NTDs). In order to reduce the frequency of NTDs and their resulting disability, the United States Public Health Service recommends that: All women of childbearing age in the United States who are capable of becoming pregnant should consume 0.4 mg of folic acid per day for the purpose of reducing their risk of having a pregnancy affected with spina bifida or other NTDs. Because the effects of higher intakes are not well known but include complicating the diagnosis of vitamin B12 deficiency, care should be taken to keep total folate consumption at less than 1 mg per day, except under the supervision of a physician. Women who have had a prior NTD-affected pregnancy are at high risk of having a subsequent affected pregnancy. When these women are planning to become pregnant, they should consult their physicians for advice.