The infants were divided into groups based on whether their birth occurred before or after January 1, 2001.

The findings are reported in Environmental Health Perspectives.

Consistent with their own previous report, the authors found that both birth weight and length decreased as umbilical cord levels of chlorpyrifos rose. A similar relationship was observed for diazinon. In contrast, propoxur, another insecticide, was only weakly linked with decreased birth length.

The average weight difference between babies with the highest and lowest levels of pesticide exposure was 186.3 grams, the investigators report.

Infants born after January 1, 2001 had substantially lower pesticide exposure levels than those born before this date, the researchers report. Moreover, the levels present in the less-exposed group seemed to have no effect on growth.

"This human study confirms the developmental impact, shown previously in animal studies, of these insecticides," Whyatt said in a statement. "It also demonstrates the positive effect of the federal ban, which has substantially reduced exposures and benefited human health."