Traffic pollution was identified as a significant problem
Exposure to traffic pollution could affect the development of babies in the womb, US researchers have warned.
They found the higher a mother's level of exposure in early and late pregnancy, the more likely it was that the baby would not grow properly.
The study, published in the Journal of Epidemiology and Community Health, looked at 336,000 babies born in New Jersey between 1999 and 2003
UK experts said much more detailed research into a link was needed.
The researchers, from the University of Medicine and Dentistry in New Jersey, used information from birth certificates and hospital discharge records.
They recorded details including each mother's ethnicity, marital status, education, whether or not she was a smoker - as well as where she lived when her baby was born.
Daily readings of air pollution from monitoring points around the state of New Jersey were taken from the US Environmental Protection Agency.
The scientists then took data from the monitoring point which was within six miles (10 km) of the mothers' homes to work out what their exposure to air pollution had been during each of the three trimesters of pregnancy.
It was found that mothers of small, and very small, birth weight babies were more likely to be younger, less well educated, of African-American ethnicity, smokers, poorer, and single parents than mothers with normal birth weight babies.
But, even after these factors had been taken into account, higher levels of air pollutants were linked to restricted foetal growth.
Two kinds of pollution produced by cars - tiny sooty particles and nitrogen dioxide - were found to have an impact.
Particulate matter is produced from vehicle exhausts and can lodge in the lungs. Fine particles, such as PM 2.5s, which penetrate deep into the lungs, have been linked to deaths from heart and respiratory diseases.
The risk of a small birth weight baby rose significantly with each increase in particulate matter of four micrograms per metres squared, during the first and third trimesters of pregnancy.
Similarly, the risk of a very small birth weight baby rose significantly with each 10 parts per billion increase in nitrogen dioxide.
Writing in the Journal of Epidemiology and Community Health, the team led by Professor David Rich, said: "Our findings suggest that air pollution, perhaps specifically traffic emissions during early and late pregnancy and/or factors associated with residence near a roadway during pregnancy, may affect foetal growth."
They say it is not clear exactly how air pollution might restrict foetal growth.
But they add previous research suggests that air pollution might alter cell activity, or cut the amount of oxygen and nutrients a baby receives while in the womb.
Professor Patrick O'Brien, of the UK's Royal College of Obstetricians and Gynaecologists, said: "This is an interesting study because it flags up a possibility of a link.
"But I think it needs to be looked at again in more detail because of the probability of confounding factors.
"The researchers ruled out smoking and social-economic background - other factors which are linked to small babies - but there are many other factors, such as diet, which could have an effect."
Professor O'Brien added that future research into the effects of pollution should be careful to check if babies are born small because their parents are small, and to ensure pregnancies are dated from scans, where this study did neither.