Unsettling association: dental X rays linked to low-birth-weight babies.

Getting dental X rays while pregnant might increase a woman's risk of giving birth to a low-birth-weight baby, a new study suggests.

Physicians try to minimize diagnostic X rays for pregnant women because radiation is most damaging to fetal and other rapidly dividing cells. When X rays in areas away from the abdomen are necessary during pregnancy, doctors shield the womb with a lead apron. Nevertheless, medical X rays have been linked to low-birth-weight babies.

Dental X rays had never been correlated with low birth weight. Previous research, however, pointed to the neck's thyroid gland as an unintended target of dental X rays. Thyroid hormone regulates metabolism, growth, and other functions, and its disruption during pregnancy could affect the development of the fetus, one hypothesis holds.

To test the effects of dental X rays on pregnancy, Philippe P. Hujoel of the University of Washington in Seattle and his colleagues analyzed dental and birth records of women in an insurance program between 1993 and 2000. The researchers identified 1,117 women who delivered low-birth-weight babies during that time and compared them with 4,468 women who had normal-birth-weight babies.

The records show that the women who had small babies--less than 5.5 pounds--were more likely to have received dental X rays totaling a dose of at least 0.4 milligray during pregnancy than were women who delivered normal-birth-weight babies. Specifically, 1.9 percent of the women with low-birth-weight babies had received this much radiation, whereas only 1.0 percent of the other women had, the scientists report in the April 28 Journal of the American Medical Association. A woman receiving about seven bitewing dental X rays would be exposed to 0.4 mGy of radiation, says Hujoel, a dentist and epidemiologist. Women should inform dentists and physicians if they are pregnant or are trying to become pregnant, he says.

"This is going to get the attention of the dental community," says endocrinologist James E. Haddow of the Foundation for Blood Research in Scarborough, Maine. The researchers didn't measure thyroid function, so they have no data specifically on the X rays' effects on that gland, he notes. Nevertheless, Haddow says, the speculation that thyroid function is altered is "not unreasonable."

As a rule of thumb, medical professionals X-ray pregnant women only "when there is a reasonable expectation of a health benefit," says S. Julian Gibbs, a dentist and radiation biologist at Vanderbitt University Medical Center in Nashville. In dentistry, he recommends X rays "only if necessary to manage urgent problems that cannot wait until [after a baby's] delivery."