"The effect is strong, which is important," Dr. Radek Bukowski of the University of Texas Medical Branch in Galveston, a researcher on the study, told Reuters Health.

The reduction in risk was greatest among women who had been taking folate for a year or longer before getting pregnant, he explained, while the preventive effects of supplementation were most powerful for preventing the earliest -- and riskiest -- preterm births.

Right now, Bukowski noted in an interview, the US Public Health Service recommends that every woman who is capable of getting pregnant take 400 micrograms of folate every day, but only about 40 percent of women of childbearing age actually do so.

Infants born early (before 37 weeks of pregnancy instead of the normal 40 weeks) run the risk of a host of health and developmental problems, with the risks greatest for babies born the earliest.

Women who deliver their babies preterm have low levels of folate in their blood, the researchers note in their report, but it has not been clear whether folate supplementation might influence preterm birth risk.

To investigate, Bukowski and his team looked at 34,480 pregnant women taking part in a study of Down syndrome screening methods. About 20 percent had been taking folate supplements for a year or more, 36 percent had been taking them for less than a year, and 44 percent of women weren't taking them at all.

There were 160 women, or 0.5 percent of the study participants, who delivered their babies before 32 weeks of pregnancy, while 1,658 (4.8 percent) delivered before 37 weeks.

Use of folate supplements for a year or more reduced the risk of delivering a baby at 20 to 28 weeks by 70 percent. Women who had been taking the supplements for less than a year were still half as likely to deliver their babies very early as women who didn't take supplements.

For deliveries between 28 and 32 weeks, the risk was 50 percent lower for women on supplements for a year or longer and 30 percent lower for women taking them for a shorter time. But there was no relationship between folate supplement use and risk of delivering a baby between 32 and 37 weeks' gestation.

The mechanism by which folic acid might prevent preterm birth is unknown, Bukowski said, although it is possible that supplementation could prevent women from developing infections of the uterus, which are associated with early preterm birth.