(CNN) Water fluoridation has been hailed by the US Centers for Disease Control and Prevention as one of the top great public health achievements of the 20th century, but a new study raises questions about its role as a potential neurotoxin in utero.

The study , published in the journal JAMA Pediatrics on Monday, found that increased levels of fluoride exposure during pregnancy were associated with declines in IQ in children. Previous research has made similar findings, but this is the first such study to evaluate the effect of fluoride on populations receiving what the US Public Health Service considers optimal levels of 0.7 milligrams of fluoride per liter of drinking water, such as in the United States and Canada.

The authors of the new study assessed 601 Canadian mother and child pairs, tracking the fluoride exposure of 512 of the mothers by looking at the average concentration of fluoride in urine samples taken throughout their pregnancies as a proxy for prenatal fluoride exposure. The authors also estimated the mothers' daily fluoride intake by surveying their beverage intake, including tap water.

Between the ages of 3 and 4, all children born from the studied mothers were tested for IQ.

The authors found that for each additional 1 milligram per liter in concentration of fluoride in a mother's urine, there was a 4.5-point drop in IQ in males. The study did not find such a significant association in female children, nor did it examine why boys were more significantly affected.