Associations were found for mothers living within a 9-square mile area around the home. That is, fetal death is more likely among mothers who are living within a 9-square mile area in which commercial pesticide spraying takes place during pregnancy. Bell et al. Increases in risk were observed for all 5 categories. Because both data sets included information on place and time, Bell et al .
Most epidemiological studies depend upon recall.
This makes them vulnerable to vagaries of memory and it also means the asessment of exposure and outcome are not truly independent. Bell et al . have avoided this problem. Toxicological studies have clearly established that the timing of exposure during fetal development is crucial to determining the outcome. But the data are usually not available in sufficient detail to allow the most vulnerable period in human development, weeks 3-8, to be examined separately. Studies are at best usually able to study the effects of during different trimesters, and often cannot consider timing at all, other than simply that exposure took place sometime during pregnancy.