Each year since 1998, a campaign to boost young women's intake of a B vitamin called folate has saved an estimated 1,000 American babies from early death or lifelong disability. But the drive to prevent neural-tube defects in newborns is stalled, sliding backward down the road to public-health victory.

It's been just eight years since American food manufacturers, complying with new federal guidelines, began adding folic acid — a readily usable form of folate — to all wheat, rice and corn products that bore the label "enriched." The new rules quickly translated into a nationwide increase in the folate levels of women in their childbearing years and a marked decline in babies born with birth defects such as spina bifida and anencephaly — abnormalities of the spinal cord and brain. By 2002, those birth defects had been driven down by 32 percent from levels of a decade earlier, according to the Centers for Disease Control and Prevention. At the time, the March of Dimes declared the news "very encouraging."

Now, it seems, even that first sign of progress is eroding — an apparent victim of dietary shifts, obesity and the stubborn resistance of women in their childbearing years to taking a multivitamin. In a report issued Jan. 5, the CDC found that among women in their childbearing years, blood folate levels had declined 16 percent by 2004 from levels recorded in 2000.

If folate intake continues to decrease at this rate, experts warned, neural-tube defects in developing babies could begin to rise again, especially among those born to white and Hispanic women, whose folate levels had risen substantially. And slow progress in getting black women to get more folate could be erased altogether.

"It's a bit frustrating," says Jennifer Howse, president of the March of Dimes. "You like to think you're gaining ground and not losing ground in something so important." Howse says the March of Dimes expects to join other groups in petitioning the Food and Drug Administration to double levels of folate in enriched grains.

Adriane Griffen, chairwoman of the National Council on Folic Acid, says it's particularly frustrating not to be able to sell Americans on the benefits of folate when it's so readily available and appears to have a growing list of benefits. In recent years, adequate folate levels have been linked to lower incidence of cardiovascular disease. And a study published recently in the Archives of Neurology found that adult men and women with the highest intake of folate were least likely to develop Alzheimer's disease, suggesting that the B vitamin may protect the brain as it ages.

Experts believe that if all women who may become pregnant took in 400 micrograms daily of folic acid, both through vitamin supplements and from legumes, liver and leafy green vegetables, the incidence of spina bifida in the U.S. could be reduced by 70 percent. To minimize spina bifida risk, women should be getting that much folate even before pregnancy — and the fact that roughly half of pregnancies are unplanned underscores the importance of getting folate well before conception.

But just one-third of women of childbearing age — down from 40 percent a few years ago — take a vitamin supplement with folate, according to the March of Dimes. Cost, lack of awareness and a widespread belief among Hispanics that a vitamin supplement could cause weight gain, all have held down folate supplementation in pill form.

CDC experts believe that many factors may have contributed to the decline in folate levels, including the trends toward low-carbohydrate diets and toward whole-grain foods not fortified with folate. Women on low-carb diets avoid most of the enriched grain products that, since 1998, have had folic acid added under a federal mandate.

In addition, researchers surmised that the growth of obesity in the population of American women of childbearing age could be driving down folate levels. Even widespread folic acid supplementation may not be enough to bring blood folate levels of overweight women up to desirable levels. From 1988 through 1994, 21.8 percent of women ages 17 to 49 were considered obese; by 1999-2000, that level had jumped to 32.3 percent.

Dr. Michael Lu, an obstetrician and public-health expert at the University of California, Los Angeles, says doctors continue to miss opportunities to get the message to women early and often.

"The American College of Obstetricians and Gynecologists and CDC and other national organizations have been recommending that we see every routine visit of a woman of childbearing age as an opportunity to provide preconception counseling and care. I think we can all do a lot better at that," Lu says.