Some households appliances could produce fields
Electromagnetic fields (EMFs) are associated with the transmission, generation and use of electricity.
Low-frequency fields can be produced by electric appliances such as hairdryers, vacuum cleaners and vehicles such as trams and trains.
The Kaiser Foundation Research Institute in Oakland, California has produced a study suggesting a strong link between these fields and miscarriage.
However, other scientists dispute whether this is the case.
High level exposure to EMFs has previously been implicated in childhood leukaemia.
But the peak levels recorded in this study are well below the recommended exposure limit of 1,600 microteslas.
Above this level, EMFs can induce electric currents in the body, which leads to localised heating.
Monitoring
Dr De-Kun Li, a reproductive epidemiologist who led the study asked 1,063 women in San Francisco who were in the first 10 weeks of pregnancy to wear a meter on their waists for a day.
The meter measured magnetic field levels in microteslas every 10 seconds.
It was found women exposed to EMFs of 1.6 microteslas or more were nearly twice as likely to miscarry as women not exposed to such strong fields.
The 622 women who said the study had covered a typical day, those who were exposed to high peak fields were three times as likely to suffer a miscarriage.
Dr Li said: "That's another confirmation that the effect is due to EMF."
He suggested the EMF peaks could cause miscarriages by subtly disrupting cell-to-cell communication.
However, Dr Li and his team did not look at what was producing the EMFs.
Other factors also influence miscarriage risk. A 30-year-old has a 5% risk, while for a woman in her mid-forties it rises to 50%.
Dr Li said his study broke new ground in EMF research because of its focus on peak rather than average exposure.
"My study opens a new chapter for these EMF effects. Not just for miscarriages, but for other health effects," he said.
A previous study by doctors at the California Department of Health Services looking at the effect of low-frequency EMFs on miscarriages was inconclusive.
When data was reanalysed to look at peak exposure, doctors found women exposed to peak EMF levels greater than 1.4 microteslas were nearly twice as likely to miscarry.
'Wrong interpretation'
David Savitz, an epidemiologist at the University of North Carolina in Chapel Hill, said: "The studies really represent state-of-the-art research into the causes of pregnancy loss".
But he said the researchers' interpretation of their findings may be wrong.
"Both studies found a reassuring lack of association for the most well-established measures of magnetic field exposure, that is average magnetic fields."
Dr Savitz added that the correlation between exposures to EMF peaks and an increased risk of miscarriage may be due to other factors, such as women who have a healthy pregnancy are more likely to suffer from nausea.
This, he said, may make them more likely to stay at home and do less, and thus also reduce their exposure to magnetic fields.
But Dr Li said his results still stood when risk factors such as drug use, carrying heavy loads and previous induced abortions were taken into account.
Dr Michael Bracken, an epidemiologist at Yale University, added: "There are numerous ways of measuring these fields, and one worries that if you do it enough times, then you are going to find positive associations.
"There's a real risk in these things getting over-interpreted and scaring the dickens out of people."
Dr Michael Clark of the National Radiological Protection Board said: "This work is interesting and will be studied carefully.
"However, the effect could be due to something other than magnetic fields. These problems need to be considered carefully before any firm conclusions are drawn.
"A few years ago there were studies which resulted in scare stories about VDUs and miscarriages which turned out to be mistaken when confounding factors were analysed independently".
The research is published in the New Scientist, and in the journal Epidemiology.