MONDAY, Dec. 6, 2010 (HealthDay News) -- Children exposed to cell phones in the womb and after birth had a higher risk of behavior problems by their seventh birthday, possibly related to the electromagnetic fields emitted by the devices, a new study of nearly 29,000 children suggests.
The findings replicate those of a 2008 study of 13,000 children conducted by the same U.S. researchers. And while the earlier study did not factor in some potentially important variables that could have affected its results, this new one included them, said lead author Leeka Kheifets, an epidemiologist at the School of Public Health at the University of California at Los Angeles.
"These new results back the previous research and reduce the likelihood that this could be a chance finding," said Kheifets. She stressed that the findings suggest, but do not prove, a connection between cell phone exposure and later behavior problems in kids.
The study was published online Dec. 6 in the Journal of Epidemiology and Community Health.
In the study, Kheifets and her colleagues wrote that further studies are needed to "replicate or refute" their findings. "Although it is premature to interpret these results as causal," they concluded, "we are concerned that early exposure to cell phones could carry a risk, which, if real, would be of public health concern given the widespread use of the technology."
The researchers used data from 28,745 children enrolled in the Danish National Birth Cohort (DNBC), which follows the health of 100,000 Danish children born between 1996 and 2002, as well as the health of their mothers. Almost half the children had no exposure to cell phones at all, providing a good comparison group.
The data included a questionnaire mothers completed when their children turned seven, which asked about family lifestyle, childhood diseases, and cell phone use by children, among other health-related questions. The questionnaire included a standardized test designed to identify emotional or behavior problems, inattention or hyperactivity, or problems with other children. Based on their scores, the children in the study were classified as normal, borderline, or abnormal for behavior.
After analyzing the data, the researchers found that 18 percent of the children were exposed to cell phones before and after birth, up from 10 percent in the 2008 study, and 35 percent of seven-year-olds were using a cell phone, up from 30.5 percent in 2008. Virtually none of the children in either study used a cell phone for more than an hour a week.
The team then compared children's cell-phone exposure both in utero and after birth adjusting for prematurity and birth weight; both parents' childhood history of emotional problems or problems with attention or learning; a mother's use of tobacco, alcohol, or drugs during pregnancy; breastfeeding for the first six months of life; and hours mothers spent with her child each day.
The investigators used the last two variables -- breastfeeding and hours spent each day with the child -- as a proxy for the kind of attention mothers gave their young children. According to the study, this was partly to determine whether a mom who spent a lot of time talking on a cell phone during pregnancy or later might be less attentive to her children -- something that might also be linked to behavior problems in her offspring.
"If breastfeeding and time spent with children are good measures of mother's attention, then we believe that our results do not support inattention as a likely explanation for the observed association," the researchers wrote.
The research did find an intriguing association between children's exposure to cell phones and their behavior.
Compared to children with no exposure to cell phones, those exposed both before and after birth were 50 percent more likely to display behavior problems, the study found. Children exposed to cell phones in the womb, but not after they were born, showed a 40 percent higher risk of borderline behavior problems. And those not exposed to cell phones before birth, but who were using them by age seven, were 20 percent more likely to have behavior problems.
One expert on child development who was not involved in the study commented favorably on its design.
"The study's methodology was rigorous and responsible. The researchers took into account as many possible variables as they could, given the limitations of the data set," said Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at the Steven and Alexandra Cohen Children's Medical Center of New York in New Hyde Park.
More than 285 million Americans no use cell phones, according to the Cellular Telecommunications and Internet Association. Some studies have raised concern that the radiofrequency energy from cell phones may pose a risk to human health, but the association between cell phone use and health problems, including cancer and brain tumors, hasn't been conclusively proven.
In the past few years, new sources of radiofrequency energy, such as wireless networks and radio-frequency identification (RFID) tags used to track products, collect tolls on highways, and speed up checkout lines--have become increasingly widespread, the study said.
While there's no reason for pregnant women to avoid using their cell phones, "precautionary measures might be warranted," Kheifets said. A simple way to reduce radiofrequency exposure is to use a cell phone's speaker mode or a headset to place more distance between your body or head and the phone, she said.
Dr. Adesman agreed. "The most conservative and perhaps prudent approach would be for both pregnant women and very young children to minimize their cell phone exposure," he said. "The risks seem to be small, but nonetheless, based on this study, they're hard to dismiss."
For more on cell phones and health, go to The National Institute of Environmental Health Sciences .