Both long-term and short-term exposure to ambient air pollution have been linked with increased risk for cardiovascular morbidity and mortality. However, few studies have examined possible associations between air pollution and gestational hypertension. Gestational hypertensive disorders are a leading cause of perinatal and maternal mortality, accounting for 2-8% of all pregnancy complications. The current study will examine associations between ambient air pollution and gestational hypertension in Halifax, Nova Scotia, using a retrospective cohort design. Information on gestational hypertension will be provided by the Nova Scotia Perinatal Atlee database which includes approximately 5,000 births per year from 1988 onward. Air pollution will be assessed using land-use regression (LUR) models and regulatory monitoring data collected by the National Air Pollution Surveillance (NAPS) network. LUR modeling will provide household level estimates of maternal exposure, while NAPS data will be used to assess pollution concentrations at the community level for the study period. Air pollution data will be linked to the Perinatal Database based on six-digit postal code (LUR) and date (NAPS) in order to estimate the risk of developing gestational hypertension. The results of this study will help to characterize risks associated with exposure to air pollution during pregnancy.