A study carried out at Indiana University and reported in the medical journal Acta Paediatrica found that birth defect rates in the US were highest in women who conceived their children in the spring and summer. This time period is also when there is an increased level of pesticides in the surface water. The researchers found a strong association/correlation between the birth defects in women conceiving/having their last menstrual period in the months of April, May, June or July and elevated levels of nitrates, atrazine, and other pesticides in surface water during the same months. Many of these chemicals use is banned in Europe, as they are suspected to be harmful to developing embryos, but their use is permitted in the US. The study relied on findings by the US Geological Survey, the EPA, and other agencies on the seasonal variations of the chemicals in surface water.
Paul Winchester professor of clinical pediatrics at the Indiana University School of Medicine, the first author of the study says, "Elevated concentrations of pesticides and other agrochemicals in surface water during April through July coincided with a significantly higher risk of birth defects in live births conceived by women whose last menstrual period began in the same months."
Indiana University News Release