Significant evidence suggests that the risk of birth defects is higher if the mother's diet lacks sufficient folic acid.

The US Food and Drug Administration has required the B vitamin to be added to certain grain products since 1998 in order to reduce rates of neural tube defects.

Initial assessment of the campaign in 2001, based on birth registry data, indicated that incidence had gone down by 19 per cent, a significant reduction, but lower than originally predicted.

A new study, reported in this month's issue of Obstetrics and Gynecology (103:474-479), suggests that the first evaluation of the effects of fortification may have been underestimated as it did not take into account a number of cases diagnosed during pregnancy that led to abortion.

Researchers at St Luke's Roosevelt Hospital in New York examined blood levels of maternal alpha-fetoprotein (AFP), a test for neural tube defects.

Data on AFP levels in 61,119 patients undergoing maternal serum screening at a large commercial laboratory were compared between two groups: before mandatory supplementation in the United States in 1997 and after mandatory supplementation in 2000.

Patients with high AFP values fell 32 per cent between the two periods.

"Our results help to validate the decision to fortify food with folic acid, which represents a highly successful public health policy for primary prevention of birth defects," write the researchers.

Folic acid fortification programmes have also been carried out in Chile and Canada but in Europe governments have been reluctant to implement similar initiatives.