<p>Air pollution is one of the most relevant threats to health worldwide. This study utilizes a combination of birth certificates for all births occurred in the capital city of Spain in 2019 and official registers of measurement stations in the 21 administrative districts. It systematically maps the influence of exposure to NO₂, as a proxy for air pollution, during pregnancy, on three birth outcomes: low birth weight, prematurity, and high birth weight. The study distinguishes between windows of susceptibility (exposure in different trimesters of gestation) and across levels of socio-economic resources of the mother. Pollution increases all birth risks in the third trimester. Findings for the second trimester are mixed. In the first trimester, exposure is associated with a lower risk of low birth weight, likely reflecting live-birth bias on characteristics that correlate with healthy birth outcomes. Even though pollution generally worsens perinatal outcomes for all, we find that mothers with more resources are able to partly offset the adverse consequences of exposure, at least up to a threshold. Socio-economic status can, therefore, be considered a moderator of the influence of air pollution on perinatal health.</p>