Air pollutants - in particular fine particulates found in traffic fumes and industrial air pollutants - along with traffic density increased the risk of low birth weight and reduced average head circumference of babies born at term, research has shown.
The study, drawn from data on 74,000 pregnant women in 12 European countries, gathered between 1994 and 2011 and published in the Lancet Respiratory Medicine, estimated concentrations in the air of nitrogen oxides and fine particulates at home addresses.
Traffic density on the nearest road and total traffic load on all major roads within 100 metres of the residence was also recorded.
Researchers estimated that for every increase of five micrograms per cubic metre in exposure to fine particulate matter - emitted by sources including diesel engines and coal-fired power stations - during pregnancy, the risk of low birth weight at term rose by 18%.
This increased risk remained at levels below the existing European Union annual air quality limit of 25 micrograms per cubic metre.
The study authors estimated that if levels of fine particulates were reduced to 10 micrograms per cubic metre, 22% of cases of low birth weight among term deliveries (low weight was classed as less than 2.5kg) could be prevented.