An Australian study has found that mothers in areas moderately polluted by carbon monoxide gave birth to children who were an average two ounces lighter, the Herald Sun reported Wednesday.

Lead author Gavin Pereira said "there was a decrease in optimal birth weight of 0.49 percent," based on an expected baby weight of 7.7 pounds.

More than 3,000 women were included in the research undertaken in Perth and published Tuesday in the Australian and New Zealand Journal of Public Health.

The study looked at carbon monoxide levels in three metropolitan areas. An association was found in only one area.

Pereira said they took the child's birth weight and then worked out their expected size under optimal conditions.

A computer model worked out the concentration of pollution at the mother's address.

"We'd expect the effects might be greater among more vulnerable populations," Pereira said. "Traffic emissions could simply tip them over the edge."

International studies into fetal growth and pollution have had mixed results.

"Our study was done in an Australian context," Pereira said. "The levels of traffic-related air pollution is quite low so to find an effect is quite surprising."

He said two ounces was about half the effect of smoking during pregnancy.

"So in that context, the weight difference is not that small, particularly as larger populations are exposed," he said.

Pereira said to minimize the contribution of traffic emissions, "use public transport, ride your bike, walk more and drive less."

Click here to read more on this story from the Herald Sun.