After adjusting for other possible causes of low birth-weight babies, such as maternal smoking, University of Washington scientists found that exposure to four "bitewing" dental X-rays more than doubles a woman's risk of having a baby weighing less than 5.5 pounds. And more X-ray exposure, such as full-mouth dental X-rays, increases the risk even more.
The risk was greatest for full-term babies at this higher radiation -- nearly four times the risk of weighing less than 5.5 pounds.
Still, researcher and periodontist Philippe Hujoel, DDS, PhD, -- along with other experts not involved in the study -- tell WebMD that it's too early to say there's a clear danger from getting dental X-rays during pregnancy, a practice that he says is typically avoided.
"'Danger' is an overstatement, because most dentists will not take dental X-rays from the moment they know a woman is pregnant," Hujoel tells WebMD. "But our findings point to a big challenge: What about women who visit their dentists and get dental X-rays when they may not be aware they are pregnant?"
Get Dental Exam Before Pregnancy
In his study, published in this week's TheJournal of the American Medical Association, Hujoel and colleagues reviewed records of more than 1,100 women who gave birth to a babies weighing less than 5.5 pounds. They also examined the infants' birth certificates. These infants were compared to nearly 4,500 normal-weight newborns.
About 10% had dental X-rays performed while they were pregnant, and Hujoel suspects that many -- along with their dentists -- were unaware they were expecting when they had the dental X-rays.
"If anything, our study suggests that if you are considering becoming pregnant, you should have a dental checkup prior to becoming pregnant," he says. "But if you suspect you may be pregnant, you should alert your dentist to this fact, or to be safe, not get dental X-rays unless they are absolutely essential."
The dilemma: During pregnancy, women are at much higher risk to develop gum problems such as gingivitis, and evidence indicates that women with gum disease during pregnancy are seven times more likely to have a baby born too early and too small. The inflammation caused by gum disease is believed to cause certain chemicals to be released in the bloodstream that affect the baby's birth weight and delivery.
Damage Done in Skull?
Still, even the American Dental Association recommends that pregnant women avoid elective dental X-rays until after delivery. "And when dentists must give X-rays to pregnant women, they usually have them wear two lead shields to protect the fetus," says Hujoel.
He notes that his study is now the third to show a possible connection between low birth-weight babies and low-dose radiation. And one of the theories is that the danger comes from radiation exposure to the woman's thyroid, pituitary, or hypothalamus glands -- rather than radiation directly to the fetus or woman's reproductive organs. These glands produce the hormones necessary for pregnancy and normal fetal development. In his current study, Hujoel looked at thyroid radiation exposure rather than radiation exposure to the fetus and the woman's reproductive organs.
"Our research was a population study and didn't examine the possible cause-and-effect, but the damage from dental X-rays may be in the skull," he tells WebMD. "That's why our findings suggest that this is something we should take a closer look at."
When his study was conducted from 1993 to 2000, very few pregnant women getting dental X-rays wore lead "thyroid shields" that protect the neck -- and thyroid -- from radiation. Now, thyroid shields are routinely used, and in fact, recommended by the ADA for all patients "whenever possible" in addition to traditional abdominal shielding with lead aprons.
Avoidance Is Best
Two other experts not involved in Hujoel's study call his findings "interesting" and agree that it warrants further study. For now, both say women who even suspect they may be pregnant should avoid dental X-rays.
"The impact of this paper is that it makes you stop and think, we should probably study this more closely, but I don't think women should be hysterically upset by this finding," says Laura Riley, MD, assistant professor of obstetrics and gynecology at Harvard Medical School and a spokeswoman for the American College of Obstetricians and Gynecologists.
"We generally advise women not to get dental X-rays during pregnancy unless there's an imperative reason for them -- and there usually isn't," she tells WebMD. "What is much more common, and what I suspect is the reason for the association in this study, is that people go to the dentist for regular care and get an X-ray before they know they are pregnant."
This view was echoed by Diane Ashton, MD, associate medical director of the March of Dimes. "We always recommend that whenever possible, pregnant women should avoid X-ray radiation," she says. "And this study points to the importance of women visiting their dentists before becoming pregnant."