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Link between stillbirth and air pollution 'inconclusive'

11:30am Wednesday 25th May 2016 content supplied byNHS Choices

"Air pollution may raise risk of stillbirth and pregnant women should consider leaving cities, say scientists," The Daily Telegraph reports.

This is somewhat radical advice given the study that prompted the headline produced no significant or conclusive results.

Stillbirth is when a baby dies before birth, but after 24 weeks of pregnancy. There are about 3,600 stillbirths every year in the UK. It is a rare but devastating outcome, and it can be difficult to know why it's happened.

Possible risk factors include infection during pregnancy, maternal smoking, maternal alcohol consumption, or having twins or multiple pregnancies. Often there is no obvious reason why a stillbirth happened.

Scientists don't know whether air pollution is linked to stillbirth. This study was carried out to summarise all the research on the subject so far. But the results are still unclear.

The pooled risks from the different studies showed a small increase in the chances of stillbirth if a woman lived in an area with raised pollution levels. But the increases in risk were so small that they could be down to chance.

While air pollution is clearly not good news for anyone's health, and governments should do all they can to reduce it, this study does not prove that it causes stillbirth. Impractical and unrealistic advice that pregnant women should move out of cities does not help anyone.

Where did the story come from?

The study was carried out by researchers from the University of Oulu, Finland, and the University of Cape Coast, Ghana, and was funded by the University of Oulu. 

It was published in the peer-reviewed journal Occupational Environmental Medicine on an open-access basis, so you can read it for free online.

The Telegraph and the Daily Mail both led on comments from one of the researchers that it would be "wise advice" to tell a pregnant woman to move to a greener area, without discussing how realistic or practical such advice actually is for most mums-to-be.

The news stories also fail to explain that the findings of this study were not statistically significant, meaning they could have been the result of pure chance.

The Independent and the Daily Mirror give more cautious views of the research and include comments from other experts, which balance their reporting.

What kind of research was this?

This was a systematic review and meta-analysis of observational studies, including cohort studies and case control studies aiming to gather evidence to see whether there may be a link between air pollution and stillbirth.

Systematic reviews are good ways of summarising the state of evidence on a topic, but they are only as good as the studies they include.

There is always a possibility with observational studies that other confounding factors - such as the health and lifestyle of the individual woman - could bias the results.

What did the research involve?

Researchers searched for studies that looked at air pollution, including a wide range of air pollutants, and stillbirths.

They included observational studies that gave information about mothers' estimated exposure to pollution (based on where they lived) and pregnancy outcomes.

They then pooled the data for different types of pollutants to see whether any of them were linked to a raised risk of stillbirth.

Most of the studies used data from air pollution monitoring stations and death certificates. Most balanced the results for confounding factors, such as the women's age and health.

Some adjusted their results to take account of the effects of other types of pollution, although most did not. Some adjusted for factors like the time of year and weather, which can affect pollution concentrations.

The researchers carried out a meta-analysis of the effect of each of six types of pollutant on the risk of stillbirth. The studies covered 11 types of pollutant, but there was not enough comparable information to do a meta-analysis on all types.

What were the basic results?

None of the six pollutants studied showed a clear risk of stillbirth. The pollutants included were:

  • sulphur dioxide
  • nitrogen dioxide
  • carbon monoxide
  • course particulate matter (PM10)
  • fine particulate matter (PM 2.5)
  • ozone

All the pollutants were linked to an increased risk when levels were higher than average, but this raised risk was too small to be sure it was not down to chance - in other words, it was not statistically significant.

In each case, the results' "95% confidence intervals" included the possibility that the raised pollution levels had no effect on risk of stillbirth.

This was true for each of the pollutants studied at every stage of pregnancy. The results showed the effect of stage of pregnancy differed from one pollutant to another, so in some the possible risk was higher in the first trimester and in others it was higher in the third trimester.

How did the researchers interpret the results?

The researchers say they found "suggestive evidence" that air pollution is a risk factor for stillbirth.

They say pregnant women "should be aware" of this risk, but that the main action required is by governments to reduce pollution levels.

They do not state in the paper itself that pregnant women should move to the countryside.

Conclusion

Pregnancy can be an anxious time for women - well-meant but alarming advice about possible risks to your unborn baby is not always helpful.

It's difficult to know what to make of a paper with inconclusive findings, like this one. As one expert says: "A reasonable headline for a press release on this work could have been 'Air pollution and stillbirth - we still don't know whether they are linked'."

The quote comes from Professor Kevin Conway, professor of applied statistics at the Open University, who concludes: "I don't think these new findings should be a serious cause for concern for individual pregnant women - if there is an increased risk of stillbirth, this review indicates that the increase is pretty small."

To put the risk into context, several of the pollutants studied were associated with a non-significant risk increase of around 2%. The non-significance means there's no evidence for a link, but even if there is one, it seems the risk increase from air pollution is likely to be very small.

Compare to this the findings of a previous systematic review, which found that secondhand smoke exposure increased stillbirth risk by 23% - and this time it was a significant link.     

However, Professor Conway and other experts agree that pollution and the potential risk of stillbirth are important topics to investigate, and future studies should be carried out to look at this area.

While the study doesn't show that pollution definitely causes stillbirth, it doesn't rule out the possibility.

One issue that needs to be addressed in future research is an accurate assessment of how much pollution individual women breathe in.

The studies assessed women's pollution exposure based on where they lived in relation to the nearest air quality monitoring station.

For some women, that was up to 25km away, so the levels monitored at the station may not reflect the quality of the air women were breathing.

Other studies have shown that just moving one street back from a busy road can make a big difference to your exposure to pollution.

We also don't know enough about the women's lives - where they worked, whether they travelled away from their homes, or what the air quality was like in their houses or workplaces.

Another major problem with the study is that even if scientists did show a strong link to pollution, we don't know whether this might have been caused by other confounding factors.

For example, people living in more polluted areas might have poorer health for other reasons, such as taking less exercise or having less money to spend on healthy food.

Finding out whether air pollution might be a cause of stillbirth is not easy. It's good that scientists are doing this research and making an effort to find out about the effects of air pollution. So far, however, we don't have enough reliable information to know its effects for sure.

The researchers' suggestion that pregnant women should consider moving to the countryside, as reported by the media, cannot be supported based on the evidence seen here. Aside from the impracticalities, moving house while pregnant could add unneeded stress during a pregnancy.

The most effective steps you can take to reduce your risk of having a stillbirth are to avoid smoking and drinking and be cautious of sources of infections known to be harmful.  

Summary

"Air pollution may raise risk of stillbirth and pregnant women should consider leaving cities, say scientists," The Daily Telegraph reports. This is somewhat radical advice given the study that prompted the headline.

Links to Headlines

Air pollution may raise risk of stillbirth and pregnant women should consider leaving cities, say scientists. The Daily Telegraph, May 25 2016

Pregnant women 'should consider moving to the countryside' because air pollution may raise the risk of stillbirth, doctors warn. Daily Mail, May 25 2016

Air pollution could increase risk of stillbirth, research suggests. The Independent, May 25 2016

Stillbirth risk increased by exposure to air pollution caused by car and industrial emissions, warn experts. Daily Mirror, May 24 2016

Links to Science

Siddika N, Balogun HA, Amegah A, et al. Prenatal ambient air pollution exposure and the risk of stillbirth: systematic review and meta-analysis of the empirical evidence. Occupational and Environmental Medicine. Published online May 24 2016

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