Living by a busy road can increase the risk of a miscarriage by more than 50 per cent. The study of 250,000 pregnant women found that air pollution is more dangerous to their unborn baby than smoking. It discovered there was a direct link between the number of “missed” – or silent – miscarriages and levels of pollutants caused by burning fossil fuels. These included particles called PM2.5s, sulphur dioxide (SO2), ozone and carbon monoxide. Lead author Professor Liqiang Zhang, of Beijing Normal University, said: “We find the risk is associated with rises for all four pollutants – and it increases with higher concentrations.”

For instance, a hike of 10 micrograms per cubic metre in sulphur dioxide made mumsto-be up to 41 per cent more prone to miscarry. Above this, the risk soared by 52 per cent. Pregnant women living and working in big cities were advised to protect themselves and their unborn babies by wearing masks, for example. Prof Zhang said: “This means the risk increase is not linear but becomes more severe the higher the pollutant concentration.”

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The study, published in Nature Sustainability, is based on health records of 255,668 women in Beijing who gave birth between 2009 and 2017. China’s capital has a miscarriage crisis and is notorious as one of the most polluted cities in the world. In total, 6.8 per cent suffered a miscarriage where the body doesn’t recognise the foetus has died in the first three months. This was compared with data from air monitoring stations near their homes and workplaces.

Prof Zhang added: “Pregnant women, or those who want to become pregnant, must protect themselves from air pollution exposure, not only for their own health but also for the health of their foetuses.” Last month a Belgian team detected soot in the placentas of all the women they studied after they gave birth. Prof Zhang said toxins could pass through the placenta and attack the immune system of a developing foetus. A 2016 report by the Royal College of Physicians and Royal College of Paediatrics and Child Health said toxins from diesel cars were fuelling a health crisis that kills 40,000 Britons a year. It said smog from busy roads and toxins from industrial emissions were linked to premature births, stillbirth, miscarriage, low birth weight and organ damage.