The study lacked data on whether babies born to X-ray-exposed mothers developed any problems associated with low birth weight, including lung ailments and delays in physical or mental growth.
Still, Dr. Michael Fleming, president of the American Academy of Family Physicians, called the study "potentially very significant because it really changes the information that we've believed all these years."
While doctors and dentists usually are cautious about taking X-rays during pregnancy, the academy has told pregnant women that medical and dental X-rays are safe.
Fleming said the study will "make us take a closer look at the data."
Similar findings have been reported in babies born to women exposed during childhood to radiation for cancer treatment. X-rays generally involve much lower radiation doses.
The study, which appears in Wednesday's Journal of the American Medical Association, involved about 4,500 women who gave birth while enrolled in a dental insurance plan in Washington state between 1993 and 2000.
A total of 1,117 low birth weight babies, or those weighing less than 5 pounds, 8 ounces (2.5 kilograms), were born to study participants. Women who had had dental X-rays during pregnancy faced about double the risk of having a low birth weight baby born either prematurely or full-term, and more than triple the risk of having a full-term underweight baby.
There was no link found between X-rays and the smallest babies, those born at less than 3 pounds, 4 ounces (1.46 kilograms).
Only 21 women who had low birth weight babies had dental X-rays, all in the first trimester, when they might not have known they were pregnant.
The study's lead author, Dr. Philippe Hujoel, a professor of dental public health services at the University of Washington, said more research is needed to determine if dental X-rays really are the culprit.
Hujoel said it is unclear how dental X-rays might affect fetuses but theorized that the radiation might cause subtle changes in the functioning of the mother's thyroid gland, in the neck. Previous studies have found an increased risk of low birth weight babies in women with mild thyroid disease, he said.
The researchers lacked information on why the women received X-rays. Hujoel said that while the X-rays were probably for routine checkups, they might have been prompted by conditions that could also increase the risk of low birth weight babies.
In the meantime, Hujoel said, the results should not discourage pregnant women with dental emergencies such as bad toothaches from seeking appropriate care, including X-rays. The risks of such problems might outweigh any dangers from the X-rays, Hujoel said.
Dr. Sally Cram, a Washington, D.C.-area dentist and spokeswoman for the American Dental Association, said the ADA advises dentists to avoid giving pregnant women X-rays during the first trimester if possible.
All patients, pregnant or not, should be given protective aprons and collars that cover the upper body and neck, Cram said.
She said the study reinforces the importance of taking care of any dental problems before pregnancy.