For the new study, reported in the Journal of Pediatrics, researchers looked at rates of neural tube defects in South Carolina from 1992 to 2009.

Infants born in South Carolina have historically had a higher rate of neural tube defects compared with the U.S. average. But during the study period, the rate of "isolated" neural tube defects (not accompanied by any other birth defect) fell from 1.4 for every 1,000 births and fetal deaths, to about 0.6 per 1,000.

And from 1998 to 2005, the average rate of spina bifida and anencephaly -- which account for most neural tube defects -- was 0.69 per 100,000. That was identical to the national average.

Folic acid would appear to take the credit, according to Dr. Roger E. Stevenson and colleagues at the Greenwood Genetic Center in South Carolina.

But folic acid may only do so much.

Obesity and type 2 diabetes in the mother are two other factors linked to a higher-than-average risk of neural tube defects. And in this study, increasing use of folic acid did not eliminate the risk associated with diabetes.