The rate of spina bifida and anencephaly birth defects has fallen by more than one-third since the addition of folic acid to the nation's enriched flours, rice and pastas in 1998, according to a study released today.

The study, published in the journal Pediatrics, prompted a renewed call from some scientists and health advocates for the Food and Drug Administration (FDA) to double the required fortification levels to further reduce the rate of the birth defects.

"We're not at maximum prevention," said Jennifer Howse, president of the March of Dimes. "We would like the FDA to reconsider this matter, hold hearings and act as soon as they can."

Other scientists, however, said not enough is known about the consequences of enriching food with folic acid and cautioned that even rare side effects could affect a significant number of people when the entire population is receiving the vitamin through food.

"No one's really looked," said Barry Shane, professor of nutrition at the University of California, Berkeley.

For instance, folic acid can mask the symptoms of vitamin B-12 deficiencies, which are common in the elderly and can lead to neurological problems.

Spina bifida and anencephaly, known as neural-tube defects, arise when the spinal column of a developing embryo does not properly close during the first weeks of pregnancy.

The defect causes paralysis in spina bifida and fatal brain deformation in anencephaly.

In 1991, scientists demonstrated that the risk of the birth defects could be significantly reduced by giving expectant mothers a synthetic version of folic acid.

The natural form of the vitamin is found in green leafy vegetables, whole grains and citrus fruits, although it is less easily absorbed by the body.

The government recommendation is for women of childbearing age to take 400 micrograms of folic acid each day.

Because the vitamin is needed so early in pregnancy, by the time many women discover they are pregnant it is too late for a folic-acid supplement to help prevent the defects.

After much debate among health researchers, the FDA mandated that starting in 1998, 140 micrograms of the vitamin be added to each 100 grams of grains that are labeled as "enriched."

A higher level was eventually added to flour.

In 2001, the federal Centers for Disease Control and Prevention (CDC) reported that U.S. rates of spina bifida and anencephaly had fallen by almost 20 percent.

The new study, also conducted by the CDC, examined birth-defect records from 20 states and Puerto Rico from 1995 to 2002.

The database, which covered more than 11 million births, turned up 4,468 cases of spina bifida and 2,625 cases of anencephaly.

Dr. Godfrey Oakley, a research professor at Emory University's Rollins School of Public Health and one of the main advocates of folic-acid fortification, said the findings showed that the FDA should require a higher dose.

Other research has suggested that as much as 75 percent of all neural-tube defects could be prevented by a higher level of folic-acid fortification, he said.

But other scientists said they were uncertain whether the move would lower the birth-defect rate much more.

Dr. Sonja Rasmussen, a CDC clinical geneticist and a co-author of the paper, said women of childbearing age can ensure they are getting 400 micrograms of folic acid each day by taking multivitamins, folic acid supplements or fortified breakfast cereals.