MEDICINE / PREGNANCY : Heat Exposure Increases the Chance of Birth Defects, Researchers Find
Women exposed during early pregnancy to heat from a sauna, hot tub, electric blanket or fever face a significantly greater risk of having a baby with a birth defect affecting the brain and spinal cord, researchers reported Thursday.
In a study of 22,775 pregnant women, the researchers found that women who had been exposed to heat three or more times in the first two months of pregnancy were six times more likely than other women to carry a fetus with a so-called neural tube defect.
In general, one to two babies per 1,000 born in the United States have a neural tube defect, the most common of which is spina bifida. Many more fetuses are believed to be affected but do not survive to birth.
“Certainly in the first three months, I believe it would be judicious to avoid hot tubs and the sauna,” said Dr. Aubrey Milunsky, a Boston University researcher who presented the findings at a pediatrics meeting in New Orleans.
The causes of neural tube defects are somewhat mysterious. Hereditary, environmental and nutritional factors have been considered. Some recent studies have suggested that multivitamins or folic acid may reduce the risk of such defects.
Perhaps the worst of the neural tube defects is anencephaly, in which much of the brain is missing. Much more common is spina bifida, in which a malformed spinal column leaves some of the nervous system exposed.
Milunsky, a professor of human genetics and pediatrics, and several other researchers from Boston University and Harvard School of Public Health extensively interviewed the women over a three-year period and tracked the outcome of their pregnancies.
The researchers found that women who used a hot tub in early pregnancy were nearly three times as likely as others to have a baby with a neural tube defect. Those who used a sauna during the same period faced 2 1/2 times the normal risk.
If those women also had a fever in early pregnancy or used an electric blanket, their risk was further increased. However, exposure to an electric blanket alone seemed to have no effect. Fever alone had a barely significant effect.
Regardless of the source of the heat, the more often a woman was exposed, the greater the risk. A woman with three exposures, for example, had more than six times the usual risk.
“What we found was quite striking and not really surprising, given the background,” said Milunsky, referring to what he described as a 20-year history of suspicion about the effects of heat on a fetus and experiments linking heat to birth defects in animals.
According to Milunsky, an animal’s developing nervous system is especially vulnerable to heat in the early weeks of embryo formation. Heat may cause cell death, restrict cell proliferation, damage small blood vessels or work in conjunction with other factors.
Milunsky presented the findings at a combined annual meeting of the American Pediatric Society, the Society for Pediatric Research and the Ambulatory Pediatric Assn. His co-authors include Dr. Walter Willett of Harvard and Ken Rothman, a Boston University epidemiologist.
