Researchers at the University of Washington School of Dentistry say that women who have dental X-rays during pregnancy are three times as likely to have a low-birth-weight, full-term baby as women who did not have X-rays during pregnancy. The research, by Dr. Philippe Hujoel and colleagues, appears in the Journal of the American Medical Association.

The association between the low birth weights in full-term babies (infants who had 37 or more weeks of gestation) and diagnostic radiography performed during routine dental care requires more investigation, Hujoel said, because the explanation for the low birth weights (less than 2.5 kilograms, or 5 pounds, eight ounces) is not clear.

“We don’t 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 said. “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 birth weight.”

Hujoel notes that the results of the study should not prevent pregnant women from having diagnostic dental X-rays if they are suffering from a dental problem. Most dentists are very cautious about taking X-rays during pregnancy. Patients should always inform their dentists about any potential pregnancy so that the dentist can postpone any elective dental X-rays until after delivery. Hujoel points out that the move toward digital radiography will drastically reduce the radiation doses needed for current techniques.

“When we first noticed this association, we were very surprised,” Hujoel said. “Currently the care guidelines say that women who are pregnant are eligible to receive very low dose radiography to the head and neck, because researchers had assumed that only direct radiation to the uterus or fetus would be harmful, and that radiation to the head and neck region would not have negative effects on pregnancies. The possibility that radiation may affect neurohormonal mechanisms in the head and neck region was not considered plausible by most researchers. The findings are also surprising because the amount of radiation pregnant women were exposed to was very low and generally thought to be incapable of inducing observable health effects. The highest dose observed in our study was about the same amount of radiation exposure as flying 16 round-trips from New York to London.”