Physical Activity and Magnetic Field Exposure in Pregnancy.

Abstract

Background:
Peak magnetic field exposure was associated with increased risk of miscarriage in 2 recent studies. Reduced physical activity levels in healthy pregnancies may affect measured exposure and thus bias results.



Methods:
We recruited 100 pregnant women to wear an Actigraph accelerometer and EMDEX magnetic field monitor for a 7-day period. We evaluated the association between physical activity and magnetic field exposure (peaks and time-weighted average) using generalized estimating equations and linear mixed models.



Results:
We found a positive association between level of activity and likelihood of incurring elevated exposure in the person-day analysis, most strongly for cutpoints of 16 or 20 mG, for both working and nonworking women among whom odds ratios in the uppermost quartile ranged from 2.1 to 2.6. A positive association was found using person-minutes only among nonworking women.



Conclusion:
Physical activity may affect peak magnetic field exposure. If the early nausea and later cumbersomeness of healthy pregnancies leads to reduced physical activity, this could distort measured magnetic field–health outcome associations.



Introduction

Recent studies have suggested that peak magnetic field exposure (but not mean exposure) is associated with pregnancy loss. A prospective study of magnetic field exposure and miscarriage reported peak magnetic field exposure of more than or =16 mG based on personal monitoring early in pregnancy was associated with a rate ratio of 1.8; however, time-weighted average magnetic field exposure was not related to pregnancy outcome. A nested case–control study measured personal exposure later in pregnancy among women whose pregnancies continued and at a comparable time for women who had a loss. Time-weighted average exposure was associated with odds ratios (ORs) of 1.7 for each of the upper 3 quartiles compared with the lowest, and the odds ratios for the second, third, and highest quartiles of maximum magnetic field were 1.4, 1.9, and 2.3, respectively.



An accompanying commentary hypothesized that the association might be an artifact of collecting personal measurement data during or after pregnancy. In the prospective study, early pregnancy nausea (strongly predictive of fetal survival) might lead to reduced physical activity, reducing the likelihood of encountering environmental sources of elevated magnetic fields. During later pregnancy, increased size may reduce activity relative to women with pregnancy losses who are not physically encumbered by advanced pregnancy. In each case, women with continuing pregnancies would be less active compared with those who had a loss, with greater activity hypothesized to be associated with increased opportunity to encounter elevated magnetic fields.



There are 2 key assumptions: 1) nausea of early pregnancy and cumbersomeness in late pregnancy are associated with reduced physical activity, and 2) reduced physical activity is associated with a lower probability of encountering environmental sources of high magnetic fields. Addressing the first question would require longitudinal evaluation of symptoms and physical activity during pregnancy or a large cross-sectional study of women of varying gestational ages having different experiences with nausea. We were not able to address the link between symptoms and physical activity in this study, but evaluated the latter assumption, namely the relationship between measured physical activity and magnetic field exposure, among pregnant women.

DISCUSSION

The association between physical activity and the probability of encountering an elevated peak magnetic field (based on person-days and on person-minutes among nonworking women) supports the hypothesis that physical activity level influences peak magnetic field exposure. These findings suggest that determinants of physical activity, possibly including nausea and other pregnancy-related symptoms, would influence peak magnetic field exposure. The other components in the hypothesized scenario—whether nausea or advanced pregnancy is associated with reduced physical activity—were not evaluated fully. We are unaware of data linking nausea to physical activity, and in fact the results found by both Li and Neutra and the present study indicate that self-reported nausea at unspecified times in pregnancy is not related to peak magnetic field exposure and was not related to physical activity in this study. Analysis of symptoms on the measurement day would be required to accurately address the link between nausea and magnetic fields or physical activity. Physical activity levels decline with advancing pregnancy and evidence from the present study suggests that peak magnetic fields decline with advancing gestation as well, pertinent to the case–control study.

Our study was motivated by the studies of magnetic field and miscarriage from northern California, but our measurements differed with regard to the magnetic field sampling frequency. Because we sampled once per minute and they sampled once per 10 seconds, the prevalence of elevated peak exposure was greater in their studies because transient peak fields would be more comprehensively identified. In fact, we found that 30% of person-days had a peak of more than or =16 mG, whereas Li et al reported 74%.

We found a notable disparity in results based on person-minutes, in which the association was restricted to nonworking women, versus person-days, which did not show such effect modification. This finding implies that movement among nonworking women increases likelihood of encountering high peak magnetic fields in the active periods, but not so among working women. Perhaps the locations in which such movement is occurring differ for these 2 subsets of women, with more concentrated exposures within the work setting for working women and more diverse settings (streets, stores, other homes) among nonworking women. However, even working women who have more active days are more likely to encounter high peaks, making the contrast of results for person-minutes and person-days difficult to reconcile. Because the previous epidemiologic studies collected measurements for person-days, the results based on person-days may be more directly applicable.

These results raise concerns regarding interpretation of personal measurements of any exposure that vary over space and time in which underlying health status may influence movement. Although peak magnetic field exposures may be an extreme version of this situation, the potential for health-related distortion of exposure measurement is a general concern.