Pregnant women who live in areas with high levels of air pollution may give birth to slightly smaller babies, according to U.S. government researchers.

A new study of more than 18,000 full-term infants born in California in 2000 found that a mother’s exposure to fine-particle air pollution seemed to make a difference in her baby’s birth weight and the infant’s risk of being below average in size.

Fine particulate matter, called PM2.5 by scientists, is composed of microscopic substances such as acids, metals and organic chemicals, and can be seen in the form of a hazy sky. It is given off by the burning of fuels from sources such as cars, power plants and some industrial processes.

In the study, babies born to women who lived in areas with the highest levels of PM2.5 were 26 percent more likely to be small for their gestational age compared with infants born to women from low-pollution areas.

The absolute difference between these groups of infants was modest. Among women with the least exposure to fine-particle pollution, 8.5 percent had a baby who was small for gestational age. That compares with 9.2 percent of women with the highest pollution exposure.

Similarly, the difference between the groups as far as average birth weight was slight, noted the study’s lead author, Dr. Jennifer D. Parker of the National Center for Health Statistics in Hyattsville, Maryland.

“But the results are consistent with other research,” she said in an interview with Reuters Health. Therefore, the “body of evidence” points to an effect of air pollution on birth weight, according to Parker.

She and her colleagues report their study findings in the January issue of the journal Pediatrics.

According to Parker and her colleagues, PM2.5 levels may influence birth weight either indirectly through effects on the mother’s health or by directly affecting fetal development.

The exact reason for the link between fine-particle pollution and birth weight is not yet clear, however, Parker said.

For their study, she and her colleagues analyzed data from air pollution monitoring stations in California and looked at birth records for 18,247 full-term infants whose mothers lived within five miles of a monitoring station during pregnancy.

The women were split into four groups based on their exposure to fine particulate matter and to carbon monoxide. Average birth weight was lowest in the group with the highest exposure to particulate matter, though the difference between this group and the group with the highest average birth weight amounted to 42 grams, or less than two ounces.

A limitation of breaking exposures into four broad groups like this is that it cannot determine anything about “extreme” exposures, Parker noted. It’s unclear, she said, whether air pollution might have a greater impact on birth weight when a woman works amid car exhaust at a highway toll plaza, for example.

The U.S. Environmental Protection Agency recently released data on the nation’s first “fine particle standard.” The agency said portions of 20 states, as well as the District of Columbia, do not meet new PM2.5 air-quality standards. Information on the affected areas is available on the EPA’s Web site at http://www.epa.gov.

SOURCE: Pediatrics, January 2005.