Background: Prior estimates of the correlation between ambient air pollutants’ concentrations and perinatal health show dispersion in magnitudes, as well as positive and negative signs. These differences may be partially explained by the diverse array of methodological approaches between studies, including the set of confounders considered. Objectives: This study explores the effect of breath able particulate matter with diameter of 10 micrometers or less(PM10) on perinatal outcomes in Uruguay, a middle-income country in South America with levels of PM10 that in general do not exceed the recommended thresholds. The analyzed outcomes are: birth weight (BW), the risk of low birth weight (LBW) and the risk of a pre-term birth (PTB). Methods: We exploit the fact that in 2011 the ashes and dust resulting from the eruption of the Puyehue volcano in Chile more than doubled monthly averages of PM10 concentration levels in Montevideo, Uruguay. Using prenatal and birth data for 2010-2013, we estimate the associations between mother’s average exposure to PM10 in each trimester-of-pregnancy and perinatal outcomes controlling for a rich set of covariates. Results: We find that exposure to high levels of PM10 concentration (above 50 μg/m³for the trimester average) during the third trimester of pregnancy is associated with higher rates of low birth weight and prematurity, and lower birth weight. These effects are quite robust to different specifications, and are particularly large for pregnancies exposed to concentration levels above70 μg/m³,suggesting non-linear effects. The impact of PM10 on BW and on the rate of LBW appears to be driven primarily by an effect on prematurity. Conclusions: Exploiting a natural experiment, our study shows that exposure to high levels of PM10during the third trimester of pregnancy can trigger preterm births.