In the 10 years since Canadian food manufacturers began fortifying their grain-based products with folic acid, the rate of spina bifida and other neural tube birth defects has been cut in half, a new study reveals.

The study, published Thursday in the New England Journal of Medicine, is not the first to show that folic acid fortification can reduce rates of birth defects. But it is the largest to assess the effects since Canada introduced folic acid fortification in 1998.

The new rules made Canada one of only a few countries in the world to require manufacturers of products such as bread, cereal and pasta to add the B vitamin to their products.

Researchers led by Philippe De Wals, a professor of social and preventive medicine at Laval University collected data on almost two million pregnancies from hospitals in seven provinces between 1993 and 2002. The data included live births, stillbirths and pregnancies that were terminated due to fetal anomalies.

In all, 2,446 subjects with neural-tube defects were recorded among 1.9 million births. The researchers found that prevalence of the defects fell from 1.58 per 1,000 births to 86 per 1,000 births after the fortification rules came into effect. That's a 46 per cent reduction.

The rate of spina bifida cases dropped 53 per cent, while rates of anencephaly (in which part of the skull and brain are missing) and encephalocele (a gap in the skull that allows part of the brain to protrude) decreased by 38 per cent and 31 per cent, respectively.

De Wals said the biggest success story of folic acid fortification occurred in Newfoundland and Labrador, where birth defect rates plunged to 0.76 per 1,000 from 4.56 per 1,000 after the vitamin was added.

Neural-tube defects had been historically higher in Canada's eastern provinces than in the western provinces. After fortification, "geographical differences in rates almost disappeared," the authors note.

The rates in Newfoundland and Labrador are now similar to those in british Columibia, which saw its neural tube birth defect rates fall to 0.75 per 1,000 births, from 0.96.