Living in a fast-paced city may increase the risk of premature birth for pregnant women who suffer from asthma, according to a recent study. Exposure to air pollution during pregnancy has been associated with an increased risk of birth defects and preterm birth, but new research published in the Journal of Allergy and Clinical Immunology found that asthmatic mothers-to-be had a greater risk of giving birth prematurely if they were exposed to traffic-related pollutants just before conception.

Premature birth is a serious condition that affects about one of every 10 infants born in the United States, according to the Centers for Disease Control and Prevention. Being born prior to 37 weeks of gestation, or preterm, is associated with many long-term disabilities, such as attention deficit hyperactivity disorder, and cerebral palsy. Early birth is also the greatest contributor to infant death in the U.S. Meanwhile, asthma is a lung disease that affects an estimated 9 percent of women of reproductive age in the U.S., according to the CDC.

"Our study found that air pollution appears to add to the preterm birth risk faced by women with asthma. These findings set the stage for further studies designed to help prevent preterm birth in this at-risk group," lead author Dr. Pauline Mendola said in a statement.

Researchers from the Eunice Kennedy Shriver National Institute of Child Health and Human Development analyzed data from a sample size of more than 223,000 pregnancies delivered at 19 hospitals across the country from 2002 to 2008. They considered the mother’s asthma status and date of delivery, to figure out the critical time-window for ambient air pollution to influence preterm birth risk. They found that exposure to high levels of traffic-related air pollutants nitrogen oxide and carbon monoxide three months before conception and in early pregnancy significantly increased an asthmatic woman’s odds of having a preterm birth.

The study found exposure to 30 parts per billion of nitrogen oxide three months prior to pregnancy increased risk of early birth by nearly 30 percent for women with asthma, compared to 8 percent for women without asthma. Exposure to carbon monoxide during this critical window was also associated with a 12 percent higher risk of giving birth prematurely, but it had no effect on preterm birth risk for non-asthmatics. The last six weeks of pregnancy were also found to be a critical window for women with asthma.

Although the effects of environmental factors on preterm birth risk is well-documented, researchers say their study is the first to examine whether air pollution before conception might impact pregnancy later.

"Early environmental exposures can have significant effects on later health," Mendola explained. "In this case, it may be that early exposure to air pollution sets off inflammation or other internal stresses that interfere with embryo implantation or placental development. Those disruptions could lead to preterm delivery down the road. More research will help us to better understand the potential impact of air pollution in the months surrounding conception."

Researchers say these findings have implications for asthmatic mothers and can help reduce their risk for preterm birth. In the meantime, it is recommended that they limit their outdoor activity when the air quality is forecast to be unhealthy.

Mendola P, Wallace M, Hwang B, et al. Preterm Birth and Air Pollution: Critical Windows of Exposure for Women with Asthma. Journal of Allergy and Clinical Immunology . 2016.