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Pregnant women may have to add another activity to the list of things that can harm fetuses: the use of hot tubs and saunas. A report in the Journal of the A.M.A. suggests that excessive heat exposure in early pregnancy can increase ^ the chance of spina bifida and brain abnormalities in their offspring. Hot tubs are the worst offenders, boosting the risk from the normal 2 in 1,000 nearly threefold. Saunas just about double the risk, as do fevers above 100 degrees F. Women exposed to two kinds of heat had six times the chance of bearing afflicted children. But electric blankets had no effect.