NEW YORK (Reuters Health) - Living near a highway may raise a pregnant woman's risk of premature delivery or having a low birth weight baby and, counterintuitively, affluent moms-to-be seem to be more vulnerable to highway pollution than their less well-off counterparts.

Using the Quebec birth registry, the researchers analysed data on nearly 100,000 live births registered between 1997 and 2001 in Montreal, Canada's second largest city where highways run through affluent and poor neighbourhoods alike. The study is published in the Journal of Epidemiology and Community Health.

They found that among all mothers residing within 200 meters of a major highway were 14 percent more likely to deliver prematurely (before the 37th week of pregnancy) and 17 percent more likely to have a low birth weight baby (less than 2500 grams, or about 5.50 pounds) compared with those residing further away.

However, in the wealthiest neighborhoods, living near a highway was associated with a 58 percent increased odds of preterm birth, an 81 percent increased odds of delivering a low birth weight baby, and a 32 percent increased likelihood of delivering a "small for gestational age" baby, whereas no association was observed in other neighborhood socioeconomic categories.

"We were a bit surprised by these findings," Dr. Delissa Dereux from University of Montreal told Reuters Health. "We initially hypothesized that proximity to a highway would affect less affluent mothers to a greater extent than affluent ones. We thought that less affluent mothers living near a highway would disproportionally suffer from the double burden of economic disadvantage and pollution."

One interpretation of these counterintuitive findings, she explained, is that less affluent mothers are more likely to be exposed to common risk factors for adverse birth outcomes during their pregnancy, including active and passive smoking, poor nutrition, domestic violence, and poor access to prenatal care.

"The impact of exposure to additional risk factors, like traffic-related air pollution, among disadvantaged mothers may not be as significant as it is among affluent mothers, who are relatively protected from these well-identified risk factors," she further explained.