Researchers studying the health effects of cell phones say they have found evidence that when pregnant women use them regularly, their children are more likely to have behavioral problems.

The study, sure to renew controversy over the safety of mobile telephones, does not demonstrate that cell phone use causes the behavioral problems and does not suggest a possible way that they could.

But the researchers say their findings are worth checking out.

"It is hard to understand how such low exposures could be influential," Dr Leeka Kheifets, an epidemiologist at the University of California Los Angeles who led the study, says.

"It is just something that needs to be pursued."

Controversy still remains over the health risk from cell phones. [Photo/China Daily]

Kheifets and her team looked at data from 28,000 7-year-olds and their mothers who took part in a large Danish study that has been tracking 100,000 women who were pregnant between 1996 and 2002.

The mothers of about 3 percent of the children said they had borderline behavioral problems, and 3 percent showed abnormal behavior, such as obedience or emotional issues.

The children whose mothers used cell phones while pregnant and who also used the phones themselves were 50 percent more likely to have behavioral problems, the researchers report in the Journal of Epidemiology and Community Health.

Children whose mothers used the phones but who did not themselves use mobile phones were 40 percent more likely to have behavioral problems, they found. They found the children were no more likely to have epilepsy or delays in development.

About 5 billion mobile phones are in use worldwide. The World Health Organization, the American Cancer Society and the National Institutes of Health have found no evidence that cell phone use can damage health.

In May 2009, experts who studied 13,000 cell phone users over 10 years hoping to find out whether they cause brain tumors found no clear answer.

International researchers launched the biggest study to date into mobile phones and health in April.

Kheifets tried to account for other possible causes, such as whether women who used cell phones were different from women who did not, especially during the time of their pregnancies when cell phone use was less common than it is now.

"We looked at social status, we looked at the sex of the child, we looked at the mother's history of behavioral problems, we looked at the mother's age and stress during pregnancy and whether the child was breastfed or not," she says.

"One thought was that it was not cell phone use but mothers' inattention that led to behavior problems. While it was important, it didn't explain the association that we found."

Nonetheless, some experts questioned the findings.

"I am skeptical of these results, even though they will get a lot of publicity," says David Spiegelhalter, a professor of Biostatistics at Britain's University of Cambridge. "The authors suggest that precautionary measures may be warranted because they have 'virtually no cost', but they ignore the cost of giving intrusive health advice based on inadequate science."

Experts at the US National Institutes of Health had no immediate comment.