A recent study urges pregnant women to avoid being exposed to air pollution as this may increase the risk of stillbirths.

Researchers are calling for tougher controls on car exhausts and industrial waste emissions to reduce the dangers of air pollutants after concluding that air pollution can have dangerous effects on pregnant women.

The study was published in the Occupational & Environmental Medicine journal following a review of 13 cases on the subject. In the study, the research found that the risk is particularly higher during a pregnant woman's third trimester.

The identified air pollutants include small particulates less than 2.5 microns in diameter, PM10, nitrogen dioxide, sulphur dioxide, ozone and carbon monoxide.

"Our results provide suggestive evidence that ambient air pollution is a risk factor for stillbirth," the researchers wrote.

"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."

However, the researchers stress that further research is needed to support their evidence, according to a report in The Independent. The authors also note that there are other factors to be considered, such as alcohol use and stress, which may also increase the risk of stillbirths.

Researchers also said that air pollution rates may differ within cities, and this could influence the results.

In an editorial linked to the study, Dr. Marie Pedersen of the Center for Epidemiology and Screening at the University of Copenhagen in Denmark said that stillbirth is often neglected, and the existing evidence pointing to the effects of air pollution should warrant additional investigation.

"If the evidence of an association between ambient air pollution and stillbirth is confirmed in future studies, it would be a major public health importance," Pedersen said in a report by The Guardian.

Stillbirths occur when a baby is born dead after 24 weeks of pregnancy. There are other complex and varied reasons for stillbirths, which may include infections, pre-eclampsia or complications in the placenta.