Exposure to airborne pollutants increases the risk of "missed" miscarriages in which a fetus dies without a pregnant woman experiencing any noticeable symptoms, researchers said Monday.

Previous studies have shown a correlation between air pollution and pregnancy complications, but the new research—published in Nature Sustainability journal by a team of researchers from Chinese universities — sheds light on a little-studied impact of pollution.

The study found that exposure to higher concentrations of airborne particulate matter, as well as sulfur dioxide, ozone and carbon monoxide, was associated with a higher risk of missed miscarriage in the first trimester of pregnancy.

Among the air pollutants, carbon monoxide has the largest impact, followed by sulfur dioxide, said Lin Jintai, co-author of the research and a professor at Peking University.

"The risk increase is not linear but becomes more severe the higher the pollutant concentration," the study said.

Missed miscarriages occur when a fetus dies or stops growing in early pregnancy while remaining in the uterus and are often detected during routine ultrasound tests weeks later.

Researchers from four universities and the Chinese Academy of Sciences tracked the pregnancies of over 250,000 women in Beijing from 2009 to 2017, using measurements from air monitoring stations near the women's homes and workplaces to gauge each subject's exposure to pollution. A total of 17,497 women, or about eight percent of the survey's participants, experienced missed miscarriage.

"We found that the risk of a missed miscarriage was associated with the age of the pregnant woman and the high and low temperatures outside," Hou Kun, a doctoral student at Beijing Normal University, who was involved in the study explained. "The missed miscarriage rate is relatively high among women over 39, female farmers and blue-collar workers."

According to lead author Zhang Liqiang from Beijing Normal University, "The significance of this research is to remind pregnant women to consciously take precautions."  

"Pregnant women or mothers who want to have babies must protect themselves from the effects of air pollution, not only for their own health but also for the health of the fetus," he said.

The study's findings are "consistent with other studies of air pollution and pregnancy loss, and also with other studies documenting significant associations between air pollutants and preterm birth," Frederica Perera, a professor of public health at Columbia University who was not involved in the study, said

(With input from AFP)