Taking folic acid supplements for a year before conception reduces the risk of premature birth by at least 50 percent, researchers reported Monday.

Shorter courses of the supplement were not as effective, according to a study of nearly 35,000 women that was reported in the journal PLoS Medicine.

“We have known for a long time that folic acid will reduce the risk of neural tube defects” and other birth defects, but the discovery that it also can reduce the risk of pre-term birth “is very significant,” said Dr. Diane Ashton, deputy medical director of the March of Dimes Birth Defects Foundation, who was not involved with the study.

The finding, she added, reinforces the recommendation that “all women of childbearing age should take multivitamin supplements.”

Only 35 percent to 40 percent of women in that age group do take supplements, according to surveys conducted by the March of Dimes and other groups.

Pre-term births account for about 12 percent of all deliveries in the United States and are associated with vision impairment, mental retardation and cerebral palsy in children, as well as diabetes and cardiovascular disease later in life. The earlier the delivery, the higher the risk of complications.