Exposure to traffic pollution from living near a busy road could affect the development of babies in the womb, warn researchers.
The study, published in the Journal of Epidemiology and Community Health, found that 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.
To reach the conclusion, researchers, from the University of Medicine and Dentistry in New Jersey, looked at 336,000 babies born in New Jersey between 1999 and 2003.
The researchers used information from birth certificates and hospital discharge records. Also, the scientists 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.
Air pollution's daily readings from monitoring points around the state of New Jersey were taken from the US Environmental Protection Agency, reports The BBC.
The scientists then took data from the monitoring point which was within six miles 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, 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.
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, the study found.
Lead researcher 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."