Pregnant women are advised to avoid having medical x-rays taken to avoid harming the developing fetus. Now a study published by the Journal of the American Medical Association links dental x-rays administered during pregnancy to low birthweight babies.

Philippe P. Hujoel of the University of Washington and his colleagues studied the records of thousands of women who received dental treatments between 1993 and 2000 in Washington State. The team compared the list to birth certificate records and found that 1,117 of the patients for whom they were able to obtain dental records had delivered low birthweight infants. According to the report, expectant mothers who underwent dental radiography were three times more likely to have a full-term baby weighing in at under five pounds, eight ounces than were women who had not had dental x-rays. "We were surprised by the finding, but it does bear out previous studies that found that other types of diagnostic radiation, such as diagnostic radiation for spine problems, also were associated with low birthweight," Hujoel says.

For now, it remains unclear what causes the association. "We dont know whether radiation affects neurohormonal mechanisms in the head and neck region, such as thyroid function, or whether factors unrelated to the x-rays are to blame," Hujoel remarks. In addition, he cautions that the results do not warrant avoiding treatment for serious dental problems, although elective dental x-rays for mom should probably be postponed until after the baby arrives.