Expectant mothers are told to take all sorts of precautions when pregnant. Risky behaviors like smoking or using alcohol are typically verboten. Heavy lifting and a poor diet should be avoided, too.

But experts now say that those who are expecting also should take care when at the dentist.

According to a study first published last year in the Journal of the American Medical Association , there is a direct relationship between low birth weight and dental X-rays taken during pregnancy. In fact, the study authors asserted that if all dental X-rays were avoided during pregnancy, the incidence of small babies could be reduced by as much as 5 percent.

It has long been known that radiation from medical X-rays could cause problems with pregnancy, but there was no definitive evidence that dental X-rays posed the same risks. Unlike medical X-rays, dental X-rays do not deliver radiation directly to the reproductive organs or the growing fetus. Instead, a dental X-ray carries radiation to the upper body, site of the hypothalamus, and pituitary and thyroid glands. That would make any effect on pregnancy indirect.

Researchers in the study examined the link between X-rays and birth weight by surveying records from Washington Dental Services, a nonprofit dental insurance company in Washington state. They looked at women who had received dental treatment between January 1993 and December 2000, and compared it with data from birth certificates. They then estimated the amount of radiation each mother had received from dental X-rays, and they looked at the amount of radiation received by mothers with normal birth weight infants.

The investigators found 1,117 low birth weight infants (those weighing less than 5.5 pounds ) and 4,468 normal weight infants born within the study window. Among the women with small babies, 1.9 percent had had a higher than 0.4 milligray (mGy) radiation dose; among women with normal-sized babies, 1 percent had that high a dose.

Women who received more than 0.4 mGy had a 2.27 times greater chance of having a small baby than women who had no dental X-rays, the study found. The odds of having a small baby were 3.6 times greater for those carrying to term.

"We've been doing things for many years to protect our patients against excessive radiation," said Dr. Sally Cram, a consumer advisor for the American Dental Association and a practicing periodontist in Washington , D.C. , told HealthDay . "But women who are considering getting pregnant or who are pregnant should certainly take very seriously their oral health so as to avoid having emergency X-rays that could potentially cause harm to the fetus or the child."

The American Dental Association recommends that elective dental X-rays be postponed until after the baby has arrived. If X-rays are needed, they recommend protective aprons for the abdomen as well as "thyroid collars" to minimize exposure to radiation. Women also should tell their dentists if they are pregnant, might be pregnant or are planning to become pregnant.