London: Pregnant women exposed to air pollution may be at a greater risk of stillbirth, a new study has claimed.

An estimated 2.6 million children worldwide were stillborn at 28 weeks or more in 2015, with the wide geographical variation in prevalence suggesting that most of

these deaths were preventable, researchers from University of Oulu in Finland said.

In 13 studies, researchers found an association between exposure to air pollution - particularly during the third term of pregnancy - and a heightened risk of stillbirth.

A four microgrammes per cubic metre increase in exposure to small particulate matter of less than 2.5 micrometres in diametre (particulate matter 2.5) was associated with a 2 percent increased risk of stillbirth, researchers said.

Exposure to nitrogen dioxide, carbon monoxide, particulate matter 10 and ozone were also linked to a heightened risk, they said.

"If the evidence of an association between ambient air population and stillbirth is confirmed in future studies, it would be of major public health importance," Marie Pedersen, from the University of Copenhagen said.

"The existing evidence is suggestive of causality for air pollution and stillbirth without precise identification of the timing of exposure," researchers added.

The findings were published in the journal Occupational & Environmental Medicine.