CHICAGO (Reuters) - Pregnant women who undergo dental X-rays may raise their risk of having low birth-weight babies, researchers said on Tuesday.

The association could be related to exposing the mothers' thyroid, pituitary or hypothalamus glands to radiation, even early in the pregnancy.

"Up until now, people assumed that head and neck radiation will not have any adverse effects on pregnant women. They assumed that only direct radiation to the uterus or the fetus would be associated with adverse pregnancy outcomes," said Philippe Hujoel of the University of Washington in Seattle, who led the study.

But a seven-year review of a dental insurance company's records in the state of Washington found pregnant women who underwent extensive dental X-rays were at three times the risk of having a low birth-weight baby, characterized as weighing 5.5 pounds (2.5 kg) or less.

Some 20 percent of the 5,585 infants in the study had low birth weight.

The study, published in the Journal of the American Medical Association (news - web sites), divided women into three groups, with the highest level of radiation exposure from dental X-rays comparable to that received in four to 16 round-trip flights between New York and London.

"Since women may not always be aware of their pregnancy status, it may not be possible to eliminate all dental radiography during pregnancy, but if this goal could be achieved and if the identified association is causal, the prevalence of (term low birth-weight) infants could be reduced by up to 5 percent," the report said.