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.