In humans, adverse pregnancy outcomes (low birth weight, prematurity, and intrauterine growth retardation) are associated with exposure to urban air pollution. Experimental data have also shown that such exposure elicits adverse reproductive outcomes. We hypothesized that the effects of urban air pollution on pregnancy outcomes could be related to changes in functional morphology of the placenta. To test this, future dams were exposed during pregestational and gestational periods to filtered or nonfiltered air in exposure chambers. Placentas were collected from near-term pregnancies and prepared for microscopical examination. Fields of view on vertical uniform random tissue slices were analyzed using stereological methods. Volumes of placental compartments were estimated, and the labyrinth was analyzed further in terms of its maternal vascular spaces, fetal capillaries, trophoblast, and exchange surface areas. From these primary data, secondary quantities were derived: vessel calibers (expressed as diameters), trophoblast thickness (arithmetic mean), and total and mass-specific morphometric diffusive conductances for oxygen of the intervascular barrier. Two-way analysis of variance showed that both periods of exposure led to significantly smaller fetal weights. Pregestational exposure to nonfiltered air led to significant increases in fetal capillary surface area and in total and mass-specific conductances. However, the calibers of maternal blood spaces were reduced. Gestational exposure to nonfiltered air was associated with reduced volumes, calibers, and surface areas of maternal blood spaces and with greater fetal capillary surfaces and diffusive conductances. The findings indicate that urban air pollution affects placental functional morphology. Fetal weights are compromised despite attempts to improve diffusive transport across the placenta.