A ground-breaking Australian study, which could explain rising rates of the chronic lung condition, found pregnant women who over-used the supplement were 30 per cent more likely to have a child with asthma.

Taking a folic acid supplement is recommended for women who intend to become pregnant, as it is prevents neural tube defects during a foetus's critical first weeks.

The University of Adelaide's Robinson Institute's Associate Professor Michael Davies has uncovered an ''additional and unexpected'' consequence for women who also take the supplement during their pregnancy.

''We see a substantial proportion of women taking these folate supplements throughout pregnancy, and it may be because people think it is entirely benign,'' Professor Davies said yesterday. ''Folate is incredibly important because of its role in preventing neural tube defects (such as spina bifida) but because it is so important, and so bioactive, it needs to be treated with some respect as well.''

Professor Davies reviewed more than 550 women who had given birth, assessing their diet and supplement intake before pregnancy and then rates of asthma among their children.

For women who took the supplements according to advice before conception and not more than several weeks into their pregnancy there was no increased risk of asthma.

But women who took them throughout their pregnancy, or from 16 to 30weeks, increased their risk of having a child with asthma by about 30 per cent.

Professor Davies said having a diet rich in green leafy vegetables, certain nuts and fruits known to be sources of folate (the natural form of folic acid) did not carry an asthma risk.

The research is published in the latest edition of the American Journal of Epidemiology.

AAP