Pregnant women who live in areas with significant air pollution risk having babies of low birth weight, the largest study to date suggests.
The study, in Environmental Health Perspectives, looked at more than three million births in nine nations.
The effect was small and individuals should not be alarmed, but there was a notable impact on the population as a whole, the researchers said.
Low birth weight babies have a higher risk of health problems and death.
The majority survive but have an increased risk of developing conditions such as diabetes and heart disease as adults.
The International Collaboration on Air Pollution and Pregnancy Outcomes (ICAPPO), by Prof Tracey Woodruff and colleagues at the University California, San Francisco, focused on airborne particulate matter small enough to penetrate the human respiratory tract.
The findings indicated the relationship between birth weight and pollution was dose related - the higher the exposure, the lower the average birth weight.
Prof Woodruff said: "What's significant is that these are air pollution levels to which practically everyone in the world is commonly exposed."
Prof Kevin McConway, a statistician at the Open University said, based on the findings, if Newcastle were to halve its current particulate air pollution level it would lead to two or three fewer low weight babies out of the total 3,500 or so born in the city each year.
He said: "That sort of reduction might well be worth having, but it's not something that pregnant mothers should lose sleep over, I'd say."
Dr Tony Fletcher, senior lecturer in Environmental Epidemiology at the London School of Hygiene and Tropical Medicine, said: "The study is of excellent quality and the conclusions are clear. While the average effect on each baby is small and so should not alarm individual prospective parents, for the whole population these small risks add up across millions of people."
The Department of Environment, Food and Rural Affairs said even though air quality in the UK is "generally good, more needs to be done, especially in the cities, to reduce the harmful effects of air pollution".
