Background : The ashes and dust resulting from the 2011 eruptions of the Puyehue volcano inChile more than doubled monthly averages of PM10 concentrations in Montevideo, Uruguay. Fewstudies have taken advantage of natural experiments to assess the relationship between ambient airpollutant concentrations and birth outcomes. Objectives : In this study we explored the effect of particulate matter with diameter of ≤ 10 μm(PM10) on perinatal outcomes in Uruguay, a middle-income country in South America with levelsof PM 10 that in general do not exceed the recommended thresholds. The analyzed outcomes arepreterm birth, term birth weight, and term low birth weight. Methods : We took advantage of the sharp variation in PM10 concentrations due to the Puyehueeruptions to estimate the associations between mother’s exposure to PM10 in each trimester of preg-nancy and perinatal outcomes. We use birth registries for 2010–2013 and control for covariates, including maternal and pregnancy characteristics, weather, co-pollutants, and calendar quarter andhospital indicators. Results: A 10-μg/m3 increase in exposure to PM10 during the third trimester was associated with ahigher likelihood of a preterm birth [odds ratio (OR) = 1.10; 95% CI: 1.03, 1.19]. The associationwas robust to different model specifications, and increased with categorical exposure levels (OR forthird-trimester PM10 ≥ 70 vs. < 30 μg/m3 = 5.24; 95% CI: 3.40, 8.08). Exposures were not consis-tently associated with birth weight or low birth weight among term births, though second-trimesterexposures were associated with higher birth weight, contrary to expectations. Conclusions: Taking advantage of a natural experiment, we found evidence that exposure to highlevels of PM10 during the third trimester of pregnancy may have increased preterm births amongwomen in Montevideo, Uruguay.