Adolescent pregnancy is a worldwide public health problem. The number of this phenomenon is increasing every year. One of the reasons for this worsening is the use of substances. This research aimed to discuss adolescent pregnancy due to chemical health risks by inadequate conduct. The method applied consisted of a cross-sectional,observational design study associated with a survey on knowledge, attitudes, and practices, carried out in 42 pregnant adolescents aged 12-18 years in a large public maternity hospital in Rio de Janeiro. A structured questionnaire was applied at their prenatal visit from November 2017 to October 2018. Descriptive tables were prepared using frequencies in absolute numbers and percentages. 11 pregnant adolescents (26.19%) reported smoking 7 cigarettes per day (SD=4.51), ranging from 5 to 45 cigarettes/day. Concerning alcohol intake, 16 adolescents (38.09%) admitted to drinking at least one occasion during pregnancy, and 8 (19.05%) in abusive form. Regarding other types of drugs (marijuana, cocaine, crack), 12 (28.57%) admitted to having used them during pregnancy, and 3 (7.14%) of these reported injecting drug use. 7 adolescents (16.66%) had a relationship with an injecting drug user partner. The survey results show that the use of illicit drugs by the pregnant adolescents interviewed was 4 times higher than the prevalence recorded in the literature for people aged 12 to 17 years. In the case of cigarette use, this difference was also observed, reaching 20 percentage points.