Premature births vary with the season, but there's nothing natural about it. Preterm birth rates peak when pesticides and nitrates measurements in surface water are highest, from April through July, and bottom out when nitrates and pesticides were lowest, in August and September, a new study found. A previous finding was that birth defects peak from April through July, the same months as pesticides and nitrates reach their maximum concentrations in surface water. The rate of premature birth in the United States has risen almost a third since 1981. Here's more on the effect of endocrine disruption in child development.