Using government data on U.S. births between 1996 and 2002, researchers found that birth defect rates were highest among women who'd conceived between April and July.
During those same months, surface-water concentrations of pesticides and other agricultural chemicals generally increased based on government water-quality assessments.
The findings, reported in the journal Acta Pediatrica, point to correlations between pesticide levels and birth defects, but do not prove a cause-and-effect relationship.
However, some of the chemicals tracked in the study, such as the weed killer atrazine, are suspected of causing harm to developing embryos, the researchers point out.
"While our study didn't prove a cause and effect link, the fact that birth defects and pesticides in surface water peak during the same four months makes us suspect that the two are related," lead researcher Dr. Paul Winchester, of the Indiana University School of Medicine in Indianapolis, noted in a written statement.
Birth defects, such as cleft lip, spina bifida and Down syndrome, affect about 3 percent of U.S. newborns. Certain risk factors, including older age, smoking or drinking in mothers, have been established, but researchers are still trying to piece together the other environmental factors that might be at work.
If further studies confirm the current findings, Winchester noted, then there will be yet another modifiable risk factor for birth anomalies.
"What we are most excited about," he said, "is that if our suspicions are right and pesticides are contributing to birth defect risk, we can reverse or modify the factors that are causing these lifelong and often very serious medical problems."
SOURCE: Acta Pediatrica, April 2009.