A high temperature from fever also increased the risk, but using an electric blanket did not.
When women were exposed to more than one source of heat, excluding the electric blankets, the chance of damaging the foetus increased more than six times.
Heat was first discovered to be a cause of deformity in chickens 100 years ago, and many studies in animals have confirmed it, Dr Aubrey Milunsky says in the Journal of the American Medical Association today.
But potentially damaging effects of raised temperatures in the mothers causing the human foetus to overheat at a critical stage in its development, have been disputed. Twenty years ago doctors decided that the evidence was tenuous and the debate subsided.
Dr Milunsky, from the Centre for Human Genetics, Boston University Medical School, has re- opened the argument with his large study in which women were asked if they had taken a sauna, a hot tub (an outdoor version of whirlpool bath in a wooden tub), if they had had a fever of 37.8C or higher or used an electric blanket during the first two months of their pregnancy.
Most of the women were attending private practices in the New England area. Telephone interviews were conducted by trained nurses.
A total of 5,566 women had been exposed to at least one heat source. Of these 1,254 had taken hot tubs; 367, saunas; 1,865 had suffered fevers and 2,883 had used electric blankets. The researchers found that taking hot tubs seemed to create the greatest risk, making the chances of neural tube defects in the foetus nearly three times as high.
Saunas and running a high fever almost doubled the risk. Electric blankets did not increase the risk significantly.
Only 22 women had been subjected to all the heat sources, which was too low a number from which to assess the multiple risks.
Out of all the pregnancies, 49 ended with a neural tube defect, either spina bifida, ancencephaly (absence of a brain) or encephalocele (in which part of the brain protrudes).
'Our findings reveal an increased risk for neural tube defects among offspring of women exposed to heat in the form of a hot tub, sauna or fever, during early pregnancy.
'More confirmatory studies are needed to evaluate the effects of timing, frequency, intensity and the duration of heat exposure on the risk of NTD to support any clinical recommendation,' Dr Milunsky and his colleagues write.
The longer a woman waits to have her first baby the greater the chances of the pregnancy going wrong, say doctors from Sweden writing in the Journal. Women aged 30 and over faced a 40 per cent increased risk of suffering late foetal death compared with women aged 20 to 24.