JAMA Pediatrics, a major peer-reviewed medical journal, has published a controversial study linking fluoridated water taken during pregnancy to lower IQ results in children. Most experts considered fluoride, which is added to water in Ireland, Canada and the U.S.A., to only have beneficial effects on teeth, preventing decay, but the latest study suggests that the fears expressed by certain lobby groups may be reasonable after all.

The Daily Beast reports: A handful of earlier studies have suggested that prenatal fluoride exposure could affect neurodevelopment, but many experts considered those to be substandard. The new study, vetted by the premier medical publisher in the U.S., is seen as more rigorous, although some experts found it unconvincing, saying the results were statistically borderline and the methodology was flawed.

“When we started in this field, we were told that fluoride is safe and effective in pregnancy,” said study co-author Christine Till of York University in Toronto. “But when we looked for the evidence to suggest that it’s safe, we didn’t find any studies done on pregnant women.” They recruited 512 pregnant women from six Canadian cities and measured their exposure several ways: analyzing the amount of fluoride in their urine; looking at how much tap water and tea they drank; and comparing the fluoride concentration in the community drinking water. Then, when the women’s children were 3 or 4, the researchers gave them IQ tests and crunched the numbers to see if they could find any trends.

“We saw an association between prenatal fluoride exposure and lower IQ scores in children,” study author Rivky Green said.

Specifically, they found a 1 mg per liter increase in concentration of fluoride in urine was associated with a 4.5 point decrease in IQ among boys—though not girls. Another translation: The boys of mothers with the most fluoride in the urine had IQs about 3 points lower than the boys of mothers with the least amount. Although critics of the study pointed to the different results by gender as a red flag, when the researchers measured fluoride exposure by examining the women’s fluid intake, they found lower IQs in boys and girls. A 1 mg increase per day was associated with a 3.7-point IQ deficit among both.