Babies born to women who have taken folic acid supplements during their pregnancy are up to 30% more likely than other children to develop asthma, researchers have found.

Mothers-to-be who take folic acid when they are between 30 and 34 weeks pregnant as a health measure may inadvertently prompt the breathing condition in their offspring, a new study says.

The research, published in the American Journal of Epidemiology, claims that its findings may help explain the huge global rise over the last 50 years in asthma, especially among children, which has puzzled experts and led to speculation over the causes of the increase.

"We believe that this is the first published study in humans to demonstrate that increasing consumption of folic acid, and specifically supplemental folate during late pregnancy, significantly increases the risk of physician-diagnosed asthma in the child at 3.5 years, persistent asthma (at 3.5 and 5.5 years), and possibly asthma at 5.5 years," write the authors, led by associate professor Michael Davies of the Research Centre for the Early Origins of Health and Disease at the University of Adelaide in Australia.

Davies and his team studied the effect of 557 Australian expectant mothers' folate consumption on their child's later development of asthma by examining their diet during pregnancy and how many of their children had asthma at 3.5 years and 5.5 years.

However, while intake of folic acid supplements in later pregnancy was potentially problematic, intake of folate – the natural form of folic acid – through consumption of green leafy vegetables, certain nuts and fruits involved no risk, said Davies. Nor did folic acid intake in early pregnancy, he added.

His research showed how changing diet patterns, and especially over-use of a supplement, could play a role in explaining childhood asthma, he added.

The Department of Health recommends that women should take folic acid for a month before they conceive and during the first trimester of pregnancy, but not beyond, as a way of reducing the number of babies born with neural tube defects, notably spina bifida. While a third of mothers take it pre-pregnancy, some continue to take it throughout, either as a single supplement or as part of a range of vitamins. But Prof Terence Stephenson, president of the Royal College of Paediatrics and Child Health, said he doubted the research was very important.

"The Royal College of Paediatrics and Child Health strongly supports the use of folic acid in pregnancy. It will significantly reduce the number of children born with serious abnormalities of the brain or spine and the associated disabilities. These conditions have profound implications for the child and parents or carers," he said. "The study published this week suggesting a link between folic acid in pregnancy and subsequent increased risk of childhood asthma is not a randomised trial. It relied on a postal questionnaire with a 76% response rate. The increased risk, if there is any, is only just statistically significant. Asthma is treatable whereas spina bifida is a serious and lifelong condition."

Leanne Metcalf, of Asthma UK, said: "While asthma is a serious and longterm condition, the benefits of taking folic acid supplements in pregnancy, particularly with regards to prevention of birth defects like spina bifida, still outweigh the risk of developing asthma."

That view was also supported by Siobhan Freegard, co-founder of Netmums, a social networking site for mothers which claims to have 2.4 million users. "It's better to have the folic acid and take my chances of asthma against the really serious risk of spina bifida that low folate intake in your diet can cause. The best thing is to take folic acid before you conceive, if you can, and for the first three months, and then get folate naturally from then on through eating things such as spinach," she said.

The government is coming under pressure to introduce the mandatory fortification of flour with folic acid as a way of reducing neural tube defects.

The move is backed by the Scientific Advisory Committee on Nutrition (SACN), its official advisers, and the Food Standards Agency (FSA).