Fetal loss associated with two seasonal sources of electromagnetic field exposure.

We have studied two sources of electromagnetic fields where exposure is highly seasonal (ceiling cable electric heat and electrically heated beds). Because many potential confounders could not be addressed, we can make no valid statement about overall differences in abortion rates between those exposed and those not exposed to these seasonal sources. However, by analyzing seasonal trends, we have minimized the problem of unaddressed confounders because we did not look at a difference between the user and nonuser groups, but rather at a difference within the user group during months of higher exposure compared with months of lower exposure. Thus, each user group provides its own control. The function of the nonuser control group is simply to assure that any seasonal trend found in the user group is not just a general seasonal variation in reported fetal loss that can be found equally well in the nonuser population. Using this technique, we found that each user group reported fetal loss disproportionately often during the season when electromagnetic field exposure was increasing.