Folic acid supplements taken for a year before pregnancy can cut the risk of premature births by 70% and save thousands of lives, researchers have found.
One in three babies born under 28 weeks die, and many others face lifelong disability. Experts say the research, based on a study of nearly 40,000 women, adds weight to calls for flour and bread to be fortified with folic acid.
Doctors have long recommended that women planning a family take folic acid supplements to reduce the risk of neural tube defects, including spina bifida, which can cause profound disabilities. But the latest study shows it has other benefits.
The Royal College of Obstetricians and Gynaecologists described the study as "extremely exciting". Tim Draycott, a consultant obstetrician and gynaecologist, said: "[Premature births] are a tremendous burden on the children, the families and society ... Anything that is potentially beneficial and particularly something as safe as folic acid - we would welcome as an intervention."
Reducing the number of premature births would also ease the burden on the NHS. Cots in premature baby units cost between £43,672 and £256,248 a year, according to the National Audit Office, depending on the level of care they provide.
More than 60,000 babies are admitted to such units each year and the total cost to the NHS in England is £420m.
In the US, it has been compulsory for manufacturers to enrich cereal-based products with 140 micrograms of folic acid per 100 grams since 1998.
The RCOG recommends that flour should be boosted with 240mcg of folic acid per 100g grams to prevent brain and spinal cord defects. Draycott said fortifying foods was more likely to be successful than asking all women of child-bearing age to take the vitamin as a supplement. According to the college only about a third of women take folic acid supplements before becoming pregnant.
The latest study looked at the effect of taking folic acid - the synthetic form of vitamin B9 - before conception. The team used information from a National Institutes of Health study about the pregnancies of 38,033 women.
By comparing groups of women who had taken folic acid for different lengths of time, the researchers found the supplement could cut the risk of premature delivery by 70% for very early premature births - 20 to 28 weeks of pregnancy - and by 50% in births after 28 to 32 weeks.
The study will be presented at the annual meeting of the US Society for Maternal-Fetal Medicine today.
In 2005-06, the latest year for which figures are available, 4,150 babies were born between 20 and 28 weeks of pregnancy. Babies born this early run a high risk of developing disabling conditions such as cerebral palsy, learning difficulties, blindness and chronic lung disease.
According to Radek Bukowski, an assistant professor of obstetrics and gynaecology at the University of Texas, who led the research, the study was prompted by previous research which showed that women who gave birth early had lower levels of folic acid in their blood.
Professor Gordon Smith, an expert in obstetrics and gynaecology at Cambridge University, said: "What you can now say is that folic acid has more general effects in pregnancy, which bolsters the case for fortifying flour. A lot of pregnancies aren't planned, so most women don't take folic acid supplements because they won't be thinking about pregnancy."
Until the research is confirmed though, Draycott said the findings should be treated with caution. Large epidemiological studies of this kind are prone to throwing up spurious results if the true factor causing health benefits is linked to whether women take folic acid or not. "The difficulty is that folic acid could be a marker of other good behaviours," he said, for example the people who take folic acid might also be well off and attend antenatal classes.
In May last year the Food Standards Agency said ministers should order the mandatory fortification of food with folic acid. But the FSA has not decided whether to add the vitamin to bread or flour because of concerns that people from ethnic minorities do not eat processed bread. There is also some evidence that folic acid has adverse effects on older people with a deficiency in vitamin B12.
"This is another piece of positive research showing fortifying is a good thing," the FSA said.
Bliss, the premature baby charity, said: "This ... could signal a positive new approach for reducing the number of premature births.
"We look forward to seeing further work done in this area - anything that improves outcomes for babies and increases their chance of having the best start to life is a very welcome development."