Having dental X-rays while pregnant may increase the chance of giving birth to an underweight baby, research suggests.
Dental x-rays use low dose radiation
Previous research has suggested a link between medical X-rays and low birth weight babies - but not dental x-rays.
Scientists at the University of Washington believe scrapping dental x-rays for pregnant women could cut low weight births by up to 5%.
The research team's conclusions are published in the Journal of the American Medical Association.
A low weight baby was defined as weighing less than 2.5 kg (5lb 8oz).
The researchers examined the records of women who received dental treatment between January 1993 and December 2000.
Among women who gave birth to underweight babies, 1.9% were exposed to a relatively high (at least 0.4 mGy) cumulative level of radiation from dental X-rays.
The same level of exposure was only half as common among women who gave birth to a normal weight baby.
Overall, women who were exposed to at least 0.4 mGy of radiation from dental X-rays seemed to be more than three-and-a-half times more likely to give birth to an underweight full-term baby than women who had no dental X-rays at all during pregnancy.
Writing in the journal, the researchers, led by Dr Philippe Hujoel, said their work questioned the idea that it was safe to give pregnant women low-dose X-rays as long as they were not targeted at the reproductive organs.
They said that ending the practice of giving pregnant women dental X-rays could potentially cut the prevalence of low weight births by up to 5%.
However, they accept, as many women do not know they are pregnant when they go for dental treatment, it would be impossible to ensure that no pregnant women is exposed to radiation in this way.
Dr Hujeol said: "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.
"The findings are 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."
Other studies have shown that different types of diagnostic radiation, such as
those used to investigate spine problems, can also associated with low birth
Professor Keith Horner, a dental spokesman for the UK Royal College of Radiologists, told BBC News Online he was very surprised by the findings.
"I find it very hard to believe that this conclusion is valid because we are talking about radiation doses within the range of normal background radiation," he said.
"It is not generally considered that there is any risk other than a very tiny increased risk of cancer from dental X-rays.
"There were reports of lower birth weights among babies born after the Hiroshima nuclear bomb was dropped, but that is a universe away from the sort of doses delivered in dental radiography."
Professor Horner said it was possible that women who delivered low weight babies had general poor health, and therefore were more likely to need dental treatment.
His views were echoed by Dr Jill Meara, deputy director of the National Radiological Protection Board, who told BBC News Online: "It is difficult to think of a mechanism whereby a level of radiation similar to natural variations that we meet all the time could have a noticeable effect."
However, she said that it was probably best for pregnant women to avoid dental X-rays unless absolutely necessary.
A spokesman for the British Dental Association said: "The risk associated with dental X-rays is very small.
"Dentists take X-rays only when they deem it to be necessary as part of diagnosis, and most dentists will be more cautious about performing dental treatments on pregnant women.
"Women must tell their dentist if they are pregnant, or even if they think they may be pregnant."