Exposure to environmental and medical radiation has been associated with increased risk of delivery of low-birth-weight (LBW) infants, mediated through the hypothalamic-pituitary-thyroid axis or possibly a direct effect on the reproductive organs. Because dental radiography directly irradiates only the hypothalamic-pituitary-thyroid axis, it provides an opportunity to identify the mechanism of action. Hujoel and associates therefore conducted a case-control study to determine whether dental radiography in pregnancy is a risk factor for LBW.

The authors used insurance company records to identify women 12 to 45 years of age who had undergone dental treatment. From a pool of women eligible for dental services, a case group comprising all pregnancies resulting in LBW infants (subdivided into very LBW, preterm LBW, and term LBW) was matched with a control group of normal-birth-weight (NBW) infants. Radiation doses were calculated for each exposed pregnant woman based on dental utilization data. The analysis controlled for such confounding variables as maternal age and ethnicity.

The study included 1,117 LBW newborns and 4,468 NBW infants. Dental radiation exposure was more common among women with LBW babies than among women with NBW babies. The prevalence of women who had more than 0.4 mGy of radiation was significantly higher for term LBW babies compared with NBW babies, but not significant for preterm or very LBW babies. The adjusted odds of delivering an LBW baby after receiving a thyroid radiation dose of more than 0.4 mGy was 2.27. Lower doses meant adjusted odds of 1.20. Each 1-mGy dose increase was associated with an odds increase of 1.83. The higher dose differences did not hold for the subsets of premature or very LBW babies and were correspondingly quite strong for term LBW babies, particularly when analyzed for radiation received in the first trimester.

Given that in this study dental radiography, especially in the first trimester, was associated with LBW, particularly at term, the authors conclude that every effort should be made to avoid dental radiography during pregnancy. It is likely that the effect on birth weight is mediated through the thyroid.