Over the past decade there has been a substantial increase in evidence suggesting an increased risk of adverse pregnancy outcomes from ambient air pollution exposure. However, there is yet to be enough convincing evidence to confirm a causal link between specific air pollutants and adverse pregnancy outcomes. The objective of this project was to address the paucity of evidence from the UK on the risk from air pollution in pregnancy. The research aim was to investigate the effects of ambient air pollution on adverse pregnancy outcomes using retrospective birth outcome data from the ‘North West Perinatal Survey Unit’ (NWPSU) during the period 2004 to 2008.In addition, primarily to determine the most appropriate exposure estimation method, a prospective comparison study (n=85) was performed to compare personal measurements of nitrogen oxides (NOx) and specifically nitrogen dioxide (NO₂) with commonly used exposure estimation techniques. This study informed two further studies which quantified the effects from air pollution in pregnancy using a large retrospective cohort from the NWPSU. The first, investigated the effects of maternal residential proximity to major roads on low birthweight (LBW), small for gestational age (SGA) and preterm birth (PTB). The second, investigated the effects of NOx, NO₂, carbon monoxide (CO) and particulate matter (PM₂.₅ and PM₁₀) based on estimates from a novel spatio-temporal air pollution model and stationary monitor sites on SGA, PTB and mean birth weight change. Linear and logistic regression models were used to quantify the risk of adverse pregnancy outcomes from living in close proximity to a major road and to specific ambient pollutants. Odds ratio (OR) associations and mean birth weight change were calculated for each of the pollutants with exposure averaged over the entire pregnancy and for specific pregnancy periods to establish critical windows of exposure. Models were adjusted for maternal age, ethnicity, parity, socio-economic status, birth season, body mass index and smoking. No statistically significant associations were found between living <200m from a major road and adverse pregnancy outcomes. Based on the spatio-temporal modelled air pollution estimates, an increased risk of SGA was found in later pregnancy with NO₂ (OR=1.14, 95%CI= 1.00-1.30), CO (OR=1.21, 1.02-1.42), PM₂.₅ (OR=1.10, 1.00-1.21) and PM₁₀ (OR=1.12, 1.00-1.25). This study provides additional evidence that women exposed to high air pollution concentrations in pregnancy are at an increased risk of an SGA birth, but not for PTB. However, there was no evidence of an effect on SGA for exposures below the current legal air quality limits.