Study shows ‘safe’ air pollution levels in UK not linked to increased stillbirth risk

Exposure during pregnancy to air pollution at the current UK recommended levels is not connected to an increased risk of stillbirth, new research from a team led by the Tommy’s National Centre for Maternal and Fetal Health in Manchester suggests.

However the study, published in the British Journal of Obstetrics and Gynaecology, shows there may be a link between exposure to nitrogen dioxide (NO2) and increased stillbirth risk in the 12 weeks before conceiving a baby.

Our researchers say more work is needed to explore this possible connection.

Existing research on a possible link between air pollution and stillbirth risk varies, depending where it is carried out, the levels of air pollution exposure, and ways of measuring it. Some studies do not take into account other factors which might affect people, such as social deprivation or whether mothers' or their partners smoke.

A team from our Centre at the University of Manchester explored data from 41 maternity units in the Midlands and North of England.  

Information from 238 women who had experienced a stillbirth at or after 28 weeks of pregnancy was included in the study and compared to a group of 597 mothers with babies born alive. All women lived within 20km of an air pollution monitoring station before and during pregnancy.

All women’s exposure to ambient pollution before and during pregnancy was calculated using recorded levels of nitric oxide (NO), nitrogen dioxide (NO2 ) and fine particulate matter (PM2.5).

The findings were adjusted to take into account other factors known to be connected to an increased the risk of stillbirth, such as mother’s BMI, smoking and household income.

The only difference found between the groups – those who sadly lost their babies, and those who did not – was that, overall, those who had a stillbirth had been exposed to higher levels of NO2 in the 3 months prior to conception.  

Previous studies have also found a relationship between NO2 and NO (at similar levels to those reported in this study) in the period before conception to gestational diabetes and hypertensive disorders of pregnancy, and lower birth weight for babies.

Our team conclude that more research must be done to understand the impact of NO2 during this period and how it could be related to these conditions, which are known to increase stillbirth risk.

Our researchers, led by Centre Director Professor Alex Heazell, found that no one whose data was included in their study was exposed to air pollution above recommended levels: all the monitoring stations showed levels which are considered safe.

This may explain differences between their findings and those from some other countries which have previously reported a correlation between air pollution exposure during pregnancy and stillbirth.

Professor Heazell says:  

“Although it is clearly desirable to reduce ambient air pollution to improve general population health overall, it is unlikely that policies to achieve this will significantly affect overall stillbirth rates in the UK and similar settings.

“However, we really need more research on the relationship between stillbirth risk and air pollution exposure above recommended levels, and further exploration of the apparent impact of NO2 in the weeks before conception.”

Our Chief Executive Kath Abrahams says:

“While this study found no increased risk of stillbirth for pregnant women who are exposed to air pollution at or below UK recommended levels, it highlights the pressing need for more research into how air pollution can affect babies’ development.
“These findings from Professor Heazell and his team come just a few months after another review of evidence showing the potentially harmful effects of air pollution at every stage of life, particularly in parts of London and other places where levels have historically been relatively high.
“Just like everyone else, women and people who are pregnant or hoping to become pregnant deserve to be protected from harm caused by their environment, wherever they live.”