BACKGROUND: In-utero exposure to particulate matter with aerodynamic diameter less than 2.5 μm (PM(2.5)) is associated with low birth weight and health risks later in life. Pregnant women are mobile and locations they spend time in contribute to their personal PM(2.5) exposures. Therefore, it is important to understand how mobility and exposures encountered within activity spaces contribute to personal PM(2.5) exposures during pregnancy. METHODS: We collected 48-h integrated personal PM(2.5) samples and continuous geolocation (GPS) data for 213 predominantly Hispanic/Latina pregnant women in their 3(rd) trimester in Los Angeles, CA. We also collected questionnaires and modeled outdoor air pollution and meteorology in their residential neighborhood. We calculated three GPS-derived activity space measures of exposure to road networks, greenness (NDVI), parks, traffic volume, walkability, and outdoor PM(2.5) and temperature. We used bivariate analyses to screen variables (GPS-extracted exposures in activity spaces, individual characteristics, and residential neighborhood exposures) based on their relationship with personal, 48-h integrated PM(2.5) concentrations. We then built a generalized linear model to explain the variability in personal PM(2.5) exposure and identify key contributing factors. RESULTS: Indoor PM(2.5) sources, parity, and home ventilation were significantly associated with personal exposure. Activity-space based exposure to roads was associated with significantly higher personal PM(2.5) exposure, while greenness was associated with lower personal PM(2.5) exposure (β = -3.09 μg/m(3) per SD increase in NDVI, p-value = 0.018). The contribution of outdoor PM(2.5) to personal exposure was positive but relatively lower (β = 2.05 μg/m(3) per SD increase, p-value = 0.016) than exposures in activity spaces and the indoor environment. The final model explained 34% of the variability in personal PM(2.5) concentrations. CONCLUSIONS: Our findings highlight the importance of activity spaces and the indoor environment on personal PM(2.5) exposures of pregnant women living in Los Angeles, CA. This work also showcases the multiple, complex factors that contribute to total personal PM(2.5) exposure.