Researchers say they have found ‘suggestive evidence’ for a link between air pollution and a heightened risk of stillbirth.

An estimated 2.6 million children worldwide were stillborn at 28 weeks or more in 2015.

There is a wide geographical variation in the prevalence of stillbirths, which suggests many of the deaths would have been preventable, say the study authors.

To date, two reviews of the available evidence have pointed to a link between air pollution and stillbirth, but the strength of the association was weak.

Now, further evidence has emerged, prompting researchers to carry out a systematic review of the findings.

Thirteen studies were eligible for inclusion in the summary. The re-assessment found an association between exposure to air pollution - particularly during the third term of pregnancy - and a heightened risk of stillbirth.

Specifically, a 4 microgram-per-cubic-metre increase in exposure to small particulate matter (less than 2.5 micrometres in diameter [PM2.5]) was associated with a 2 per cent increased risk of stillbirth.

Exposure to nitrogen dioxide, carbon monoxide, PM10 and ozone were also linked to a heightened risk.

But there were big differences in study design and the type of pollutant assessed across the 13 papers, which meant many were left out of the final analysis.

Dr Marie Pedersen, of the Centre for Epidemiology and Screening, University of Copenhagen, says most of the previous studies did not take account of potentially influential factors, such as obesity, infections, alcohol, and occupation and stress, all of which have been associated with an increased risk of stillbirth.

Despite the caveats, the study authors conclude: “However, the existing evidence is suggestive of causality for air pollution and stillbirth without precise identification of the timing of exposure.”

But they add that further research is needed to strengthen the body of evidence available: “With the limited studies on the relevant topic, our review suggests strong priorities for future research,” they write.

“Stillbirth is one of the most neglected tragedies in global health today, and the existing evidence summarised by [the authors] deserves additional investigation,” writes Dr Pedersen.

“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,” she adds.

She also says that even though the size of the effect seems relatively small, the ubiquitous nature of ambient air pollution exposure suggests that exposure to it might have considerable impact on stillbirth risk at the population level.