Ambient air pollution is an environmental health problem, both nationally and worldwide. It has been linked to several adverse health outcomes, including lung cancer, acute lower respiratory infections, strokes, and an increased risk of premature death. Exposure to air pollution is responsible for about one in eight global deaths, according to the World Health Organization. It has also been linked to pregnancy outcomes such as premature birth and autism. New research published in the journal Occupational and Environmental Medicine suggests prenatal exposure to air pollution could also increase the risk of stillbirth.

“Pregnant women should be aware of the potential adverse effects of ambient air pollution, although the prevention against exposure to air pollutants generally requires more action by the government than by the individual,” study authors wrote.

Researchers carried out a systematic review of 13 studies regarding air pollution exposure and stillbirth published up to 2015; however, only five were included in the meta-analysis. After combing through these studies, researchers found an association between exposure to air pollution — particularly during the third trimester of pregnancy — and a heightened risk of stillbirth.

Stillbirth, the death of a baby before or during delivery, affects about 1 percent of pregnancies in the U.S., killing about 24,000 babies each year, according to the Centers for Disease Control and Prevention. Worldwide, an estimated 2.6 million children were stillborn at 28 weeks or more in 2015. While advances in medicine over the past 30 years have reduced the number of stillbirths, many scientists are still not sure of its causes.

Fetal growth and pregnancy outcome are determined by several factors including maternal nutrition, environmental exposures, and heredity,” researchers wrote. “The prenatal stage of life is a very sensitive period such that exposure to harmful substances can have an adverse effect on the developing fetus.”

The analysis showed that a 4 microgram per cubic meter increase of exposure to small particulate matter, including dirt and smoke, of less than 2.5 inches diameter (PM2.5) was associated with a 2 percent increased likelihood of stillbirth. Exposure to nitrogen dioxide, carbon monoxide, and ozone, which are present by way of vehicular emissions, industrial waste and cigarette smoke, were also linked to a heightened risk.

The findings provide suggestive evidence that prenatal exposure to outdoor air pollution is a risk factor for stillbirth. Although most of the existing evidence used relied on “air monitoring data, which doesn't adequately capture variations in levels within the same city,” researchers said, adding that the review is suggestive of a causal relationship between air pollution exposure and stillbirth.

However, researchers said future studies on this topic should integrate the activity level of mothers and their activity in the outdoors, whether they have had change in residence, and a mother’s occupation.

Source: Siddika N, Balogun H, Amegah A, Jaakkola J. Prenatal ambient air pollution exposure and the risk of stillbirth: systematic review and meta-analysis of the empirical evidence. Occupational and Environmental Medicine. 2016.