BACKGROUND: Maternal ambient air pollution exposure is associated with reduced birthweight. Few studies have examined the effect on growth in utero and none have examined the effect of exposure to particulates less than 2.5µm (PM) and possible effect modification by smoking status. OBJECTIVES: Examine the effect of maternal exposure to ambient concentrations of PM, PM and nitrogen dioxide (NO) for in utero fetal growth, size at birth and effect modification by smoking status. METHODS: Administratively acquired second and third trimester fetal measurements (bi-parietal diameter, femur length and abdominal circumference), birth outcomes (weight, crown heel length and occipito-frontal circumference) and maternal details were obtained from routine fetal ultrasound scans and maternity records (period 1994-2009). These were modelled against residential annual pollution concentrations (calendar year mean) adjusting for covariates and stratifying by smoking status. RESULTS: In the whole sample (n=13,775 pregnancies), exposure to PM, PM and NO was associated with reductions in measurements at birth and biparietal diameter from late second trimester onwards. Among mothers who did not smoke at all during pregnancy (n=11,075), associations between biparietal diameter and pollution exposure remained significant but were insignificant among those who did smoke (n=2700). Femur length and abdominal circumference were not significantly associated with pollution exposure. CONCLUSIONS: Fetal growth is strongly associated with particulates exposure from later in second trimester onwards but the effect appears to be subsumed by smoking. Typical ambient exposures in this study were relatively low compared to other studies and given these results, it may be necessary to consider reducing recommended "safe" ambient air exposures.