Researchers in the United States have been trying to figure out why rates of premature birth have been rising for the past several decades. In the U.S., so many children are now born prematurely that the average gestational age of all births has dropped by about a week, from 40 to 39 weeks. An infant is considered premature when it's born at 37 weeks of gestation or earlier. Prematurity is a leading cause of infant mortality.

Dr. Paul Winchester, from the medical school at Indiana University, has been researching whether environmental factors could be playing a role in birth defects and pre-maturity. Using government data on about 25 million births over a six-year period, he examined the rates of pre-maturity on a monthly basis. "We can calculate a percent of births that are premature, born in January, February and so on," he explains. "The pre-term birth rate, by month, creates a curve, if you will, and it has its peak in June and it also has its trough, the lowest part of the curve, in August and September, and then it rises again in the fall."

Winchester matched this curve with government data on levels of pesticides in drinking water. He says he focused specifically on nitrates and other chemicals, such as atrazine, which has been shown in animal studies to affect the reproductive system. "Many, many lines of evidence now have related [those contaminants] to pre-term birth in ecological studies: nitrates in the well water of Prince Edward Island, pesticides in the drinking water in various communities, have been associated with pre-term birth."

Winchester found a strong correlation between peak applications of agricultural pesticides and peak numbers of pre-term births. The numbers also dropped in months when fewer pesticides were being applied across the country. He points out, "if environment were irrelevant, then the curve should be straight across, there should be no difference in pre-term birth during any month of the year. The fact that there is a regular pattern to pre-term birth at the very least allows us to look at that month of worst risk and perhaps be more vigilant that month and to ask what could be driving it."

Winchester says his findings point to the need for more and better research on how pesticides could be affecting pre-term births. He presented his research recently at the annual meeting of the Pediatric Academic Societies, a group that includes both U.S. and Canadian researchers.