A letter published in The Lancet says there has been little change in the number of babies being born with neural tube defects (NTDs) such as spina bifida in the 1990s.

This has been the first decade during which health experts have been encouraging women to supplement their diets with folic acid pills.

Research published in the 1980s suggested women who had had one baby with an NTD were less likely to have another if they increased their folic acid intake.

Other studies showed there could be a general benefit to pregnant women from consuming more folic acid.

Government guidance on taking folic acid supplements was issued in 1991.

And in 1992 the UK Expert Advisory group said first-time mothers-to-be should take the pills.

In 1996 the Health Education Authority launched a campaign to raise awareness about the issue.

But statistics show that, although NTDs fell during the 1980s, possibly because of improved diets, this trend did not continue into the 1990s.

Six possible reasons

The Lancet letter, from Leonore Abramsky and colleagues at the Imperial School of Medicine Department of Medical and Community Genetics, speculates that this may be due to a number of reasons.

They believe, for example, that women most likely to benefit from taking the pills are not planning their pregnancies or are not aware of the benefits of taking folic acid supplements.

If they are planning their pregnancy and know of the benefits of taking folic acid they may be deciding not to take it or may not be taking it at the right time.

Other possible reasons are that women are taking supplements at the right time but are not benefiting from them or that those who would benefit were already getting increased folic acid in their diet in the 1980s.

The Health Education Authority says it is too early to judge whether its publicity campaign has worked.

The statistics only go up to the end of 1997 and the publicity campaign in 1996 and is ongoing, it says.

It adds that research on target groups shows that awareness of the benefits of taking folic acid has increased from 9% to 49% between 1995 and 1998.

No room for complacency

But HEA Folic Acid Project Manager Lucy Thorpe says: "This study does show that it is important not to be complacent. We need sustained and persistent campaigning to maintain awareness, but we can't do it alone."

She called on health workers to spread the message about folic acid.

The HEA is working with manufacturers to get them to increase folic acid in foods such as cereals and bread.

It is also planning to do research into whether vulnerable women such as those on low incomes and those from ethnic minorities have got the folic acid message.