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As there have been growing concerns about the adverse effects of air pollution on birth outcome, studies for this area has been carried out in different populations and sites. We reviewed the epidemiologic studies that evaluated the effects of air pollution on birth outcome such as low birth weight and preterm births. We identified the air pollution exposure during pregnancy was related with low birth weight and preterm birth, although there are differences among studies for the critical period of vulnerability. The biological mechanisms whereby air pollution might influence health of fetus are not clearly established. The exposure to carbon monoxide (CO) during pregnancy could increase fetal carboxyhemoglobin and result in tissue hypoxia. On the other hand, ambient particles less than 10 micrometer in aerodynamic diameter (PM10) could lead to inflammation and increase blood viscosity. Controlling for potential confounders and valid assessment of exposure are the methodological issues remained in these epidemiologic studies. In the future, more studies are needed to investigate the effect of air pollution on preterm birth or stillbirths, considering the various exposure period and the biological mechanism.