The scientists are calling for a worldwide introduction of food fortification programmes, which they claim offer a more effective means of increasing folate intake among women.
In Canada, which introduced folic acid fortification of grain-based foods in 1998, the prevalence of neural-tube defects among both unborn and newborn children has been halved and it has also had a similar benefit on incidence of the deadly childhood cancer neuroblastoma.
But Europe remains fearful of possible side-effects of a wide-ranging fortified food initiative. A two-year investigation into flour fortification by the UK's Food Standards Agency concluded in 2002 that this could mask a deficiency of vitamin B12 in elderly people.
Yet the new review argues that without such a programme, there are many factors currently preventing women from obtaining enough folic acid around the time of conception.
Dr Joel Ray at the University of Toronto, Gita Singh of McMaster University in Hamilton and Robert Burrows of Monash University in Australia reviewed nearly 50 studies conducted in about 20 countries between 1992 and 2001.
They write in this month's British Journal of Obstetrics and Gynaecology (vol 111, issue 5, p399) that reported use of folic acid prior to conception varied from 0.9 per cent to 50 per cent while supplement use around the time of conception ranged from 0.5 per cent to 52 per cent.
The research points out that the neural tube is fully developed 22 to 28 days after conception, but many women are not aware they are pregnant until after this time and starting folic acid supplements after this period is too late to realize benefits. Food fortification with folic acid, already carried out in Canada, the US, Chile and Israel, has however raised "an incredible debate.. in the United Kingdom and Europe," admitted Ray.
"There has been a heated discussion about the long-term safety of folic acid and, while no harm is evident, we are just beginning to study the effects of long-term exposure. But as a society, where's the greater good versus the lesser harm? Fortification is probably the best way to reach most women worldwide, given that not enough women take tablet supplements alone."
Consideration should be given to the practical advantages of folic acid fortification of centrally processed foods, such as wheat, corn and rice flour, while further promoting vitamin tablet supplement use and planned pregnancy, concluded the researchers.
At a meeting in January Europe's national food safety agencies acknowledged the difficulties in achieving the recommended 400 micrograms of folate through the diet and promised they would explore ways of reducing folic acid deficiency among women of child bearing age.
Products that have been fortified with folic acid or are naturally rich in folate and supplements could be further promoted by national agencies, along with increasing consumer awareness, especially in women, of the benefits of increasing their folate status, they said.
However while four studies examining the effect of mass media campaigns on periconceptional folic acid use found that rates increased significantly, by a factor of 1.7 to 7.2, in no study was the post-campaign rate above 50 per cent, according to the new review.
Women with a low level of formal education or young maternal age were less likely to take folic acid while immigrant status, a lack of partner and an unplanned pregnancy also reduced folic acid intake.