About 2500 to 3000 affected babies are born in the U.S. each year, although the rate has been declining since the early 1970s. The vitamins must be taken before the baby's neural tube forms, however. This means in the first three to four weeks of pregnancy. Ideally, they should be taken in the three months before conception and continued in the first three months after conception. It is also possible to fulfill this requirement by eating fortified bread or cereal, but many weight-conscious women may not want to eat 14 slices of bread or 4 cups of cereal daily, and most Americans do not ordinarily consume enough green leafy vegetables or dried beans to meet the requirements.
Some groups benefit more from folic-acid containing vitamins than others.
In one study, conducted by the California Birth Defects Monitoring Program, college-educated women did not benefit at all, perhaps because they already had enough folic acid in their diets. Folic acid, while reducing risk, is not a sure preventative. There is clearly some genetic component affecting a woman's chances of having a child with spina bifida or anencephaly. One possibility is a reduced ability to metabolize folic acid and therefore a need for higher than-average intake. Rates vary by ethnicity.