Medics claim that frequent exposure to magnetic fields from electrical devices may triple youngsters chances of getting the condition.
Researchers in the US conducted a 13-year study of 800 women and measured their exposure to magnetic fields during their pregnancies, and then also monitored their children.
They found that 130 children, or 20.8 per cent of study participants, developed asthma, with most of the children's cases being diagnosed before the age of five.
The study's lead researcher, De-Kun Li said: 'Pregnant women should try to keep their distance from known magnetic field sources.'
Previous studies have failed to consistently show that chronic exposure to electromagnetic fields (ranging from power lines to microwaves ovens, hair dryers and vacuum cleaners) are harmful to human health.
British experts however, still say the study - which was published in Archives of Pediatrics and Adolescent Medicine - has weaknesses.
More:Advice And Health
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