NEW YORK » Pregnant women who eat rice regularly might expose themselves and their fetuses to too much arsenic, possibly putting them at risk of premature births, researchers from Dartmouth College said.

A test found that pregnant women who ate rice in the two days before their urine was analyzed had a median level of the potentially toxic chemical 39 percent higher than those who didn’t. The study of 229 New Hampshire women was adjusted for the amount of arsenic in the water supply, according to the research published today in the Proceedings of the National Academy of Sciences.

Arsenic occurs naturally in the environment and is common in ground water. While drinking water is tested, and there are guidelines for safe levels of the element — 10 micrograms per liter — no such monitoring or regulation exists when it comes to rice and other foods, said Margaret Karagas, senior author and director of the Children’s Environmental Health and Disease Prevention Center at Dartmouth in Hanover, N.H.

"Rice is a nutritious food, so we are not making a dietary recommendation that women avoid it," Karagas said in a telephone interview. "What we would like to see is for our food to be monitored for the presence of arsenic and regulated to keep it below certain levels."

Exposure to arsenic has been connected with the development of cancer and cardiovascular illness and may be related to premature births, low birth weights and other adverse pregnancy outcomes, Karagas said.

"Developing fetuses may be more vulnerable to environmental agents," she said.

The level of arsenic in rice varies based on where the grain is grown, among other factors, Karagas said. For this reason, the researchers don’t recommend dropping rice from a pregnant woman’s diet, she said. In general, rice is more able than some other plants to extract arsenic from the environment.

"This is why it is so important to study which species and growing conditions result in higher levels of arsenic so we can avoid them," Karagas said.