Pregnant women with lower levels of folate in their blood may be more likely to give birth to a baby with low birth weight, nutrition researchers say.

Folate or vitamin B9 is found in leafy green vegetables, citrus fruits, liver, nuts, fortified wholegrain breads and cereals.

Women are advised to take the synthetic form, folic acid, before conceiving and during the first months of pregnancy to prevent birth defects such as spina bifida.

Folic acid is added to flour, cornmeal and pasta in Canada.
Folic acid is added to flour, cornmeal and pasta in Canada.

Folic acid is essential for growth and gene expression in the fetus.

Researchers at University of Newcastle upon Tyne have now linked higher levels of folate to increased birth weights, a marker for good health in infancy and later life.

About seven per cent of all babies are born low birth weight, or less than 2.4 kilograms or 5.5 pounds.

The team studied folate levels in the blood of nearly 1,000 women and their newborns.

The researchers, who also looked at lifestyle data, found women who smoked tended to have lower levels of folate in their blood, and gave birth to smaller babies.

It's thought that smoking may change the ability of cells to metabolise and store folic acid.

Taking supplements and eating folate-rich foods are the best ways to increase folic acid levels, the researchers said.

"The evidence from this study strengthens the argument for fortifying everyday foods like bread and cereals with folic acid," said study author Dr. Caroline Relton of Newcastle University's School of Clinical Medical Sciences.

The Canadian government introduced mandatory fortification of some foods with folic acid in 1998.

The study on expecting women and their newborns appears in the British Journal of Nutrition.