OBJECTIVE: To investigate the association of maternal race/ethnicity with risk factors for brachial plexus birth injury (BPBI) and quantify the proportion of excess BPBI risk due to these factors. STUDY DESIGN: This retrospective cohort study of all livebirths occurring in California-licensed hospitals from 1996-2012 included 6,278,562 maternal-livebirth infant pairs with 7762 BPBI diagnoses. Multiple logistic regression and causal mediation analyses were used to evaluate the relationship of race/ethnicity and BPBI risk factors. RESULTS: Black and Hispanic birthing-individuals were at increased risk of obesity, diabetes, and limited prenatal care utilization but decreased risk of many BPBI risk factors (shoulder dystocia, macrosomia, prolonged second stage of labor, and vaginal delivery). CONCLUSIONS: Black and Hispanic birthing-individuals were at lower risk of many strongly associated risk factors for BPBI, and these factors mediate only a small proportion of their excess BPBI risk, underscoring the importance of identifying alternative risk factors, especially drivers of demographic disparities.