The conclusion was based on the analysis of the records of 1,800 pregnant women and their babies in three metropolitan areas.An association was found in only one of these areas，Perth suburbs，an area with low industrial activity，with a mix of polluted and quiet roads.Researchers said that they took the baby's weight at birth and then worked out their expected size under optimal conditions.At the same time，a computer model was calculating the concentration of traffic emissions around the homes of mothers up to the time they gave birth.

　　Dr.Gavin Pereira，assistant professor，who specializes in the effects of air pollution on child's health，found in his previous research that Perth's traffic pollution was increasingly the risk for about seventy per cent of young kids who suffer from serious asthma(哮喘)attacks.He said the findings from that latest study were very surprising for him because the pregnant women who took part in the study did not live in heavily polluted neighborhoods.“I did not really expect to see any effect，so it was pretty surprising，although some international studies have found some associations，”said he.

　　Researchers said that their results should not create any panic but rather be a warning signal for the future.The population grows each year so just that alone will increase the level of traffic emissions.Besides，the number of vehicle kilometers travelled is expected to go up so there are plenty of reasons why the problem of air pollution is going to get worse.

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