Taking folic acid supplements for a year before conception reduces the risk of very premature birth by at least 50%, researchers reported Monday.
Shorter courses of the supplement were not as effective, according to the study of nearly 35,000 women reported in the Public Library of Science journal PLoS Medicine.
Folic acid's effectiveness in reducing the risk of neural-tube and other birth defects -- even without such a long course -- is long established. The discovery that it can also reduce the risk of preterm birth "is very significant," said Dr. Diane Ashton, deputy medical director of the March of Dimes Birth Defects Foundation, who was not involved in the study.
The finding reinforces the recommendation that "all women of child-bearing age should take multivitamin supplements," she said.
Only 35% to 40% of such women do take supplements, according to surveys conducted by the March of Dimes and other groups.
Preterm births -- before 37 full weeks of gestation -- account for about 13% of U.S. deliveries and are associated with vision impairment, mental retardation and cerebral palsy in children, and diabetes and cardiovascular disease as they grow older.
The earlier the delivery, the higher the risk of such complications.
Previous research had shown that women who deliver prematurely have lower-than-normal levels of folate in their blood. Small trials of folic acid supplements to prevent premature birth have given mixed results.
Dr. Radek Bukowski and his colleagues at the University of Texas Medical Branch at Galveston were able to screen a much larger number of women by piggybacking their research onto a National Institutes of Health study that was testing methods of screening for Down's syndrome.
"It was such a large data set, with so much information, that we were able to answer this question as well," Bukowski said.
The women were questioned during the first trimester of pregnancy about their health behaviors, including use of supplements.
The researchers found that for women who had been taking folate for at least a year before conception, the risk of birth between 28 and 32 weeks was reduced by 50%.
The risk of birth between 20 and 28 weeks was reduced by 70%.
The latter reduction is particularly significant, Ashton said, because researchers had previously found no way to reduce the proportion of preterm babies born so early.
The supplementation had no effect on the risk of preterm birth after 32 weeks, the researchers found. And beginning supplementation around the time of conception did little to reduce preterm births, even though such timing does reduce the risk of neural-tube defects.
The scientists do not know why this is so.
The U.S. has required since 1998 that flour and similar products be fortified with folic acid. Beyond that, health authorities recommend that women of childbearing age -- whether or not they plan to become pregnant -- consume an additional 400 micrograms per day.
There has been recent debate about whether even higher amounts would be more beneficial -- although some critics believe that higher amounts could mask vitamin B12 deficiency and might raise the risk of certain cancers.
But studies such as this one, Bukowski said, suggest that "dose is not as important as duration."