Pregnant women face a higher risk of miscarriage if they are exposed to elevated levels of a type of air pollution, a study suggests.
Expectant mothers exposed to higher levels of nitrogen dioxide, a gas created by petrol and diesel vehicles, are 16% more likely to lose their baby.
One researcher at the University of Utah, which led the research, described the findings as "upsetting".
It comes amid a growing concern over air pollution and its links with diseases and conditions such as strokes, dementia and autism.
The NHS says one in eight women who know they are pregnant will miscarry, while the actual figure may be higher because many women lose their child before they know they are pregnant.
Doctors analysed the records of more than 1,300 women who sought help from the University of Utah's emergency department following a miscarriage between 2007 and 2015.
The team of researchers worked out the risk of miscarriage during periods of three to seven days after a spike in the level of common air pollutants, including small particle matter and nitrogen dioxide.
Researchers found an increased risk in miscarriage for women exposed to the elevated levels of nitrogen dioxide, which equalled a rise of 16% for a 10 parts per billion increase during a seven-day window.
But the particulate matter did not significantly increase the risk of miscarriage.
Matthew Fuller, associate professor of surgery at the university, said: "The results of this study are upsetting, and we need to work together as a society to find constructive solutions."
He advised women to raise health concerns with their doctor, and suggested they limit their outdoor activity on days when the air quality is poor.
Research analyst Claire Leiser said the data only related to the most severe cases during a small window of time, and did not account for women who might have gone to their obstetric or primary care providers for support.
She said: "The results are not the whole picture."