U.S. National Institutes of Health researchers analyzed data from more than 50,000 women who gave birth in 20 hospitals in Utah between 2002 and 2010, as well as data on airborne pollutants the women were exposed to during pregnancy.
For nearly all pollutants, the women's exposure was more likely to decline over time. But between 7% and 12% of the women in the study had higher exposure to air pollution during their second pregnancy.
The increased exposure was largely from carbon monoxide (51%) and nitrogen dioxide (45%), mainly in emissions from motor vehicles and power plants; ozone (48%), which is created by combustion products and sunlight; and sulfur dioxide (41%), from the burning of fossil fuels such as coal or diesel.
"What surprised us was that among low-risk women, including women who had not delivered preterm before, the risk during the second pregnancy increased significantly when air pollution stayed high or increased," study lead author Pauline Mendola said in an NIH news release. She's a senior investigator in the epidemiology branch of the Eunice Kennedy Shriver National Institute of Child Health and Human Development.
The study was published Sept. 12 in the International Journal of Environmental Research and Public Health.
Preterm or premature birth (before 37 weeks) is a leading cause of infant death in the United States. A normal pregnancy is generally 40 weeks long.
Previous research has linked air pollution to the risk of preterm birth. But this study may be the first to connect this risk to changes in exposure between a first and second pregnancy, according to the researchers.
They said more study is needed to confirm the association, but added that reducing air pollution may help reduce the risk of preterm birth.