Exposure to traffic pollution can increase the risk of pregnant women suffering pre-eclampsia by as much as a third, an Australian study suggests.

The effect of traffic-related air pollution is even greater for women deemed to be at-risk of developing the disorder, such as indigenous women and women with diabetes.

"Modest increases in exposure were associated with a 30 per cent increase in risk — and more-so among women with other major risk factors for pre-eclampsia," says lead author Gavin Pereira, who did the research while at the Telethon Institute for Child Health Research in Western Australia.

Pre-eclampsia is a disorder that occurs only in pregnancy and post-birth and can be life-threatening for the mother and unborn child.

The causes and origins of pre-eclampsia are not well understood, but it normally develops late in pregnancy and can impact on the mother's various body functions such as the cardiovascular system, liver and kidneys.

The results of the study were published last month in the Journal of Epidemiology & Community Health.

Reduce car use

Pereira, now based at the Yale Centre for Perinatal, Pediatric and Environmental Epidemiology at Yale University, says exposure to traffic-related air pollution is practically unavoidable in an urban environment.

However, he believes there is potential for decreases in the levels of pollution to prevent some cases of pre-eclampsia and associated deaths.

"It is infeasible for pregnant women to avoid this ubiquitous exposure," he says.

"Air pollution can be present even if you cannot see it or smell it. The obvious message for the public is to reduce your reliance on your car. Use public transit and switch to active modes of transport like walking and cycling."

Nitrogen dioxide measurements

For the study, Pereira and colleagues took measurements of the nitrogen dioxide, as a marker for traffic-related air pollution around Western Australia's Perth metropolitan area in 2010.

They developed a model to predict these measurements based on the season of measurement and exposure to major roads near the measurement site.

They also undertook a retrospective study in the southwest area of Perth, Australia, which identified 23,452 pregnant women who had delivered children between 2001 and 2006.

Of this group 943, or 4 per cent, developed pre-eclampsia. Pereira used the modelling to predict the levels of the traffic-related pollutant at the residential addresses of the women in the year of their pregnancy.

The study, published recently in the Journal of Epidemiology Community Health, shows elevated exposure in the third trimester and higher average traffic-air pollutant levels across a whole pregnancy are associated with greater risk.

"Although I cannot fully explain why exposure seems more relevant in late pregnancy, it is this period when most cases of pre-eclampsia develop," says Pereira.

"My interpretation is that traffic-related air pollution is more of a precipitating or promoting cause than an initiating cause of pre-eclampsia."

"That is, some women develop pre-eclampsia independently of pollution levels, and some women don't develop pre-eclampsia irrespective of the pollution levels, but for some women air pollution could be the last straw."

Risk higher for those with gestational diabetes

He says the strongest link between traffic pollution and pre-eclampsia was for women with diabetes.

"Women with gestational diabetes have a higher risk of developing pre-eclampsia as their pregnancy progresses," Pereira said. "So, again, there is suggestion that traffic-related pollution seems to promote or precipitate pre-eclampsia in already susceptible pregnancies."

He says the study did not pinpoint exactly which chemicals in traffic emissions was causing the adverse impact.

However, he says he hopes to answer these types of questions while at Yale using chemical data from the U.S. Environmental Protection Agency and a very large cohort of approximately half a million pregnancies.