objective: to determine the prevalence of psychiatric disorders during pregnancy, the prevalence of cocaine and marijuana use during the third trimester of gestation and the socio-demographic characteristics of a population of low-income teenagers. method: one thousand pregnant teenagers were evaluated using the composite international diagnostic interview, and a socio-demographic and socio-economic questionnaire at the obstetric center of a public hospital in s？o paulo, brazil. hair sample was collected for analysis. results: of the 1000 pregnant teenagers interviewed, 53.6% were poor, 90.4% were unemployed, 92.5% were financially dependant and 60.2% dropped out of school. those using drugs during the third trimester of pregnancy were equal to 6% (marijuana: 4%, cocaine: 1.7%, both: 0.3%). those having at least one psychiatric disorder equaled 27.6%. the most frequent diagnoses were depression (12.9%), posttraumatic stress disorder (10.0%) and anxiety disorders (5.6%). discussion: unstructured families, dropping out of school, unemployment and a low level of professional training are all contributing factors to the maintenance of an unfavorable socio-economic environment in which there is a high prevalence of cocaine and marijuana use during the third trimester of pregnancy and an abnormally high incidence of psychiatric disorders.