To quantitatively assess the relationships of childhood experiences of marriage, pregnancy, and sexual violence with underage sex trafficking.Cross-sectional survey data from a population-based sample of 603 female sex workers from Tijuana and Ciudad Juárez, Mexico, were collected in 2013 and 2014, and we analyzed the data to evaluate relationships between pregnancy, marriage, and sexual violence when younger than 16 years, and child sex trafficking.Adjusted odds of child sex trafficking among those who experienced pregnancy, marriage, and childhood sexual violence when younger than 16 years in combined models were 2.8 (95% confidence interval [CI] = 1.8, 4.3), 1.7 (95% CI = 0.99, 2.8), and 1.7 (95% CI = 1.01, 3.0), respectively, relative to others (n = 603). For 89.8%, 78.0%, and 97.0% of those who had an experience of pregnancy (n = 69), marriage (n = 50), or sexual violence (n = 33) when younger than 16 years, respectively, the experience occurred before or the same year as sex trafficking.These results provide empirical evidence of modifiable risk factors for child sex trafficking that could be integrated into the prevention efforts and protocols of health clinics and governmental agencies in Mexico currently working to reduce underage pregnancy, marriage, and sexual violence.