She cautioned that she could not say if electromagnetic radiation from cell phones or any number of other factors, such as parenting styles, might explain the link between maternal cell phone use during pregnancy and childhood behavioural problems. "I would say interpret these results with caution, and everything in moderation," she said in a Skype interview.
Birks and her colleagues analysed data on more than 80,000 mother-child pairs in Denmark, Spain, Norway, the Netherlands and Korea. They found consistent evidence of increasing risk of behavioural problems - particularly, hyperactivity - in 5- to 7-year-old children the more their mothers talked on cell phones during pregnancy.
Given that there is no known biological mechanism that could lead prenatally emitted cell phone radiation to promote hyperactivity in offspring, the results were surprising, said Birks, who is a doctoral student in biomedicine at the Barcelona Institute for Global Health in Spain.
The association held firm across five countries and time periods.
Offspring of mothers who reported being on at least four cell phone calls a day, or in one cohort speaking on a cell phone for more than an hour a day, were 28 percent more likely to be hyperactive than offspring of mothers who reported being on one or fewer calls a day, researchers found after accounting for a variety of confounding variables, such as maternal age, marital status and education.
The data spanned a variety of time periods from 1996 through 2011. Only the earliest cohort, in Denmark starting in 1996, had enough women who never used a cell phone while pregnant to study women who did not use cell phones during pregnancy.
But the children of mothers who never used cell phones while pregnant had a lower risk of behavioural and emotional problems than any of the children whose mothers used cell phones, according to the report in Environment International. Dr Robin Hansen, a paediatrician and professor at the University of California, Davis in Sacramento found the report raised more questions than it answered.
"Is it something about the cellphone itself?" she asked in a phone interview. "Is it something that impacts your parenting behaviour? Those are issues that can't be answered by this study." As a paediatrician who works with children who have behavioural problems, Hansen is less inclined to consider cellphone radiation and more inclined to consider parenting styles, habits and personalities as a possible link between maternal cellphone use and childhood hyperactivity, she said. She was not involved in the study.
"Now we have to dig deeper and figure out why," Hansen said. "Is it the electronic signals that go through your brain and your body, or how it changes your interactions with your child postnatally?" American paediatricians advise parents to limit their children's screen time. But parents also need to consider how their time spent tethered to their phones takes them away from their children, Hansen said.
When parents stare at their phones and fail to respond to their kids, their children quickly learn how to get the attention they crave, she said. "It's not until you cry or you throw something or make a lot of noise, that your parents shift their attention from the cellphone to you," she said. So children learn to make a racket in an effort to pull their parents toward them and away from their devices. "It reinforces hyperactive, attention-getting behaviour," she said.