Background:In-utero exposure to fine particulate matter (PM2.5) and specific sources and components of PM2.5have been linked with lower birthweight. However, previous results have been mixed, likely due to heterogeneity in sources impacting PM2.5and due to measurement error from using ambient data. Therefore, we investigated the effect of PM2.5sources and their high-loading components on birthweight using data from 198 women in the 3rdtrimester from the MADRES cohort 48-hour personal PM2.5exposure monitoring sub-study.Methods:The mass contributions of six major sources of personal PM2.5exposure were estimated for 198 pregnant women in the 3rdtrimester using the EPA Positive Matrix Factorization v5.0 model, along with their 17 high-loading chemical components using optical carbon and X-ray fluorescence approaches. Single- and multi-pollutant linear regressions were used to evaluate the association between personal PM2.5sources and birthweight. Additionally, high-loading components were evaluated with birthweight individually and in models further adjusted for PM2.5mass.Results:Participants were predominately Hispanic (81%), with a mean (SD) gestational age of 39.1 (1.5) weeks and age of 28.2 (6.0) years. Mean birthweight was 3,295.8g (484.1) and mean PM2.5exposure was 21.3 (14.4) μg/m3. A 1 SD increase in the mass contribution of the fresh sea salt source was associated with a 99.2g decrease in birthweight (95% CI: −197.7, −0.6), while aged sea salt was associated with lower birthweight (β =−70.1; 95% CI: −141.7, 1.4). Magnesium sodium, and chlorine were associated with lower birthweight, which remained after adjusting for PM2.5mass.Conclusions:This study found evidence that major sources of personal PM2.5including fresh and aged sea salt were negatively associated with birthweight, with the strongest effect on birthweight from Na and Mg. The effect of crustal and fuel oil sources differed by infant sex with negative associations seen in boys compared to positive associations in girls.