Pregnant women who are exposed to low levels of air pollution seem to have an increased risk of giving birth before term, Australian researchers report.

Dr. C. Hansen, of the University of Queensland, Brisbane and colleagues studied data on 28,200 live births in Brisbane for the period of July 1, 2000 to June 30, 2003. The team calculated the average maternal exposure for ambient particulate matter (PM10), ozone, and nitrogen dioxide over the first three months after the last menstrual period and the last three months before birth.

There were a total of 1583 preterm births (5.6 percent), defined as gestation of less than 37 weeks, the researchers report in the British Journal of Obstetrics and Gynaecology.

A significantly higher percentage of preterm infants were born during winter. Exposure to PM10 during the first trimester was associated with a 15 percent increased risk of preterm birth. Exposure to ozone was linked to a 26 percent increased risk.

The researchers also found that the PM10 exposure effect associated with the first trimester was strongly linked to exposure during the first month after the last menstrual period.

Although overall the effect was small, and did not reach significance from a statistical standpoint, the investigators stress that the ambient air pollution concentrations were also relatively low.

SOURCE: British Journal of Obstetrics and Gynaecology, September 2006.