Fortification of grains was introduced in the US in March 1996 and made mandatory in January 1998 at a level of 140 micrograms of folic acid per 100 grams of grain.
The Centers for Disease Control and Prevention (CDC) estimates that the incidence of NTDs has declined by 26 percent since 1998, but that 2000 affected pregnancies still occur in the United States each year.
The program has been hailed as a success in some quarters. The largest general population research program carried out to date was published in last month's American Journal of Clinical Nutrition.
This looked at B vitamin status indicators in blood samples collected from 7,300 participants aged three years and over as part of the 1999-2000 National Health and Nutrition Examination Survey. It concluded that "every segment of the US population appears to benefit from folic acid fortification".
But the new study, published in the September issue of Pediatrics, is not so upbeat and has led to calls to considerably increase the fortification level.
Led by Laura Williams, the team from the CDC and the University of Alabama set out to investigate how the decline in neural tube defects varied amongst specific racial and ethnic groups between 1995 and 2002.
They studied data from 21 population-based birth defect surveillance systems, which identified 4468 cases of spina bifida and 2625 cases of anencephaly.
The prevalence of spina bifida and anencephaly was found to be highest in Hispanic and non-Hispanic white births, but a significant decline in incidence for these two races/ethnicities (26 and 34 percent respectively) was noted as the fortification program came into effect. The decline was more marked with spina bifida than with anencephaly.
In non-Hispanic black births, there was a lower incidence of NTDs to begin with, but the effect of fortification on cases of spina bifida was of only borderline significance and it was not statistically significant at all for anencephaly.
"Efforts to increase folic acid consumption for the prevention of NTDs in pregnancies among women of all races/ethnicities should be continued, and studies to identify and elucidate other risk factors for NTDs are warranted," was their conclusion.
In a strongly-worded editorial accompanying the study, Robert Brent and Godfrey Oakley wrote: "It is a tragic failure of public policy, both in the United States and around the world, that a single case of folic acid-preventable spina bifida and anencephaly occurs."
Pediatrics first published a commentary arguing for increased fortification in 2000, but according to the authors the FDA has failed to take their arguments on board.
Brent and Oakley would like to see the fortification levels doubled to 280 micrograms per 100 grams of grain. They also urged the FDA to require suppliers to add vitamin B12 to grain products, and corn flour manufacturers to enrich their products with folic acid to prevent more NTDs in Hispanic births.
Birth defects foundation the March of Dimes goes even further, recommending that the required level of folic acid be set at 350 micrograms per 100 grams of grain.
But in fact, the US fortification program is not replicated the world over. Like the rest of Europe, UK has opted not to introduce it despite pressure from scientists, on the grounds that it may impede the diagnosis of other nutrient deficiencies such as B12, which is common in the elderly.
The FDA has claimed that the level of fortification in foods means that daily intake from all sources would remain below 1mg per day, unless consumers elevate their levels with dietary supplements. This, it says, removes the risk of complicating the diagnosis of other deficiency-related conditions, like pernicious anemia.
Further fuel has been added to the argument this week with the presentation of a Norwegian study indicating that folic acid and vitamin B9 may exert no protective effect on the cardiovascular system - and may in fact contribute to an increased risk of heart attack.