For years, many communities have added fluoride to drinking water to help reduce cavities. But a new study that has found a link between fluoride levels in pregnant women and lower intelligence in their children may provide further ammunition for those who are calling that practice into question.

In the first study of its kind, investigators at the University of Toronto, McGill, the Harvard School of Public Health, and other institutions have found a link between fluoride in the urine of pregnant women and lower measures of intelligence in children.

In the study released Tuesday in the journal Environmental Health Perspectives, the researchers say they looked at 287 pairs of mothers and children in Mexico City at multiple stages of neurodevelopment. The study recruited pregnant women from 1994 to 2005, taking urine samples from the mothers during pregnancy and from their children between six and 12 years of age to assess their exposures to fluoride.

They then analyzed how fluoride in the pregnant mothers’ urine related to measures of intelligence. Those measures included the children’s verbal abilities at age four, as well as their perceptual performance, memory, and motor abilities. They were tested again for the same things between the ages of six and 12.

The researchers found a statistically significant connection between high levels of fluoride in the mothers when they were pregnant and lower scores on these intelligence measures. There was no link between the children’s own urinary fluoride levels and their test scores.

Dr. Howard Hu, the study’s lead investigator, and a professor of environmental health, epidemiology and global health at U of T’s Dalla Lana School of Public Health, says the fact that the fluoride levels in the mothers was most predictive of the drop in test scores may be due to the fact that the brains of babies develop so rapidly while they are in utero.

“This is consistent with a growing appreciation in environmental health that the growing fetal nervous system is more sensitive to exposures than a developed nervous system,” he told CTVNews.ca by phone from Sydney.

Tap water in many communities in Canada has been fluoridated for decades, in a bid to prevent cavities and improve bone health. But in recent years, research has begun to emerge suggesting that fluoride may be a neurotoxin.

“Back when they started that study, no one was worried about fluoride,” Dr. Hu says.

But he says a seminal 2007 book by the U.S. National Research Council of the National Academies described the link between the mineral and neurodevelopment.

Dr. Hu says some research has suggested the fluoride may interfere with cell messenger neurotransmitters. Studies on mice suggest it can accumulate in the hippocampus, an area of the brain important to learning and memory.

Dr. Hu notes those studies tested fluoride at much higher doses than humans would receive in fluoridated water. But he says those studies “open the door” to seeing fluoride as a neurotoxicant.

“The levels of exposure are higher. But there is now a rationale for expecting that it may also be toxic at lower levels,” he said.

The researchers adjusted their analyses for other factors that might have impacted the children’s neurodevelopment, such as the children’ birthweight, their gestational age, the mothers’ smoking history, their IQ, lead exposure, and more, but the link they found held.

“There’s always the potential that fluoride really is the proxy of some other real actor that’s playing a role. But I can’t imagine what that would be,” Dr. Hu said.

“We tested for all the things we could think of that could act on neurodevelopment. But we haven’t found anything else that was a potential confounder.”

Dr. Hu and his colleagues acknowledge that fluoridated water and toothpaste have both substantially reduced the incidence of cavities and should be acknowledged “as a public health success story.”

But they say that more research is needed on the potential adverse effects of fluoride, particularly in pregnant women and children.

“We emphasize that this is just one study. It’s a rigorous study but it’s just one study,” Dr. Hu said.