Traffic-related air pollution may increase the likelihood of pre-eclampsia in pregnant women, especially in the third trimester, new findings suggest.

In Perth, Western Australia, researchers estimated residential exposure to nitrogen dioxide—a marker for traffic-related air pollution—during pregnancy among 23,452 women. Each interquartile range (IQR) increase in nitrogen dioxide during the third trimester was associated with a 30% increase in the risk of pre-eclampsia, after adjusting for other risk factors. In addition, each IQR increase during the whole pregnancy was associated with a 12% increased risk, investigators reported in the Journal of Epidemiology and Community Health (2012; published online ahead of print).

The increased risk was more pronounced in women with pre-existing or gestational diabetes, in whom each IQR increase during the third trimester was associated with a greater than threefold increased risk in pre-eclampsia risk, in adjusted analyses. For the entire pregnancy, each IQR increase was associated with a 53% increased risk.

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Aboriginal women and women aged 20 or younger and 40 or older also had higher risks than the overall study population. Each IQR increase during the whole pregnancy was associated with a 35% and 34% increased risk in pre-eclampsia among the aboriginal women and those in the aforementioned age groups, respectively.

The researchers, led by Gavin Pereira, PhD, of the University of Western Australia in Perth, noted that the “biological mechanisms by which traffic-related air pollution may relate to pre-eclampsia are not yet well understood, partially because the condition is a multisystem disorder of unknown etiology. However, it is known that particulate matter air pollution is capable of augmenting the development and progression of atherosclerosis and may potentially contribute to hypertension.” Dr. Pereira’s team also cited research suggesting that pre-eclampsia and vascular atherosclerosis may share common pathways in relation to pollutants.

Dr. Pereira said he is unable to explain fully why exposure to air pollution seems more relevant in late pregnancy, but he noted that this is the trimester when most cases of pre-eclampsia develop. He interprets his findings to suggest that traffic-related air pollution precipitates or promotes pre-eclampsia rather than initiates it. Pre-eclampsia develops in some women independently of pollution levels, Dr. Pereira said, and it does not develop in other women regardless of pollution levels, but for some women, “air pollution could be the last straw.”

For the study, the investigators measured nitrogen dioxide levels at 22 sites across a study area of 238 km2. They selected sites to ensure representation of subregions and high, moderate, and low traffic roads. The air samplers were placed near traffic counters operated by local governments and the state.