The vitamin folate was already known to reduce the risk of birth abnormalities known as neural tube defects. But the study, published in the British Journal of Nutrition, has suggested that it can also affect the weight of newborn babies.

Increased Risk of Disability

Researchers from the University of Newcastle upon Tyne studied almost 1,000 women and their babies. Blood samples taken during routine antenatal visits were analyzed for levels of folate. Mothers with higher levels were more likely to have heavier babies, while low levels were associated with low-weight infants.

Seven percent of all babies are of low birthweight (less than 5.5 lb.), leaving them at increased risk of disability. The findings may also explain why women who smoke are more likely to have smaller babies.

Smoking can affect the ability of the cells to store folate, so that even if a woman is taking folic acid supplements, her baby may not get the benefit.

Critical Window

Dr. Caroline Relton, lead researcher on the study, said: "Many women are missing this critical window in the first few weeks of gestation, during which their baby really needs folic acid to develop and grow."

Dr. Renton said her findings strengthened the argument for folic acid to be added to bread and cereals.

"Fortifying food promises to be a more effective solution than a campaign to encourage women to take folic acid supplements," she said. "Our previous research has shown younger women and those from deprived backgrounds are less likely to take these supplements, and although some cereals are fortified, they tend to be the more expensive, brand-name products."

Add Folic Acid to Flour?

A 10-year campaign to encourage pregnant women to take folic acid supplements before and during pregnancy has not led to widespread use of the supplements. Women are advised to take 400 mg a day before conception and in the first 12 weeks of pregnancy.

Folate is essential for the production and maintenance of new cells and is particularly important when rapid cell division and growth is taking place in pregnancy.

It is found in vegetables such as spinach and broccoli, but 40 percent of pregnant women admit they had an unhealthy diet before conception.

Ministers are considering whether to add folic acid to flour. Neural tube defects fell dramatically in Canada and the US after the supplement was added to flour. But some experts fear fortification can mask anemia.