Pesticides may increase the risk of birth defects, say researchers who found that the highest rates of birth defects in U.S. babies occur among those conceived in the spring and summer, the same time that there are increased levels of pesticides in surface water.

Researchers analyzed all 30.1 million births in the United States between 1996 and 2002. They found a strong association between higher rates of birth defects among women whose last menstrual period was in between April and July and elevated levels of nitrates and other pesticides in surface water during those same months.

The data showed a statistically significant correlation between the last menstrual period and higher rates of birth defects for half of 22 categories of birth defects, including spinal bifida, cleft lip and Down syndrome. The study is published in Health Day News.