This study identified and compared the prevalence of tobacco, alcohol and illicit drug use in a large sample of low-image pregnant women, stratified by maternal age and race/ethnicity. We used a cross-sectional, population-based design and used structured interviews. All 903 patients attended university, low-risk obstetric clinics, were aged 12-41 years and were of white, black and Mexican-American race/ethnicity. The results indicate that the prevalence of tobacco, alcohol and illicit drug use during pregnancy and lifetime illicit drug use varied by age, race/ethnicity and type of substance. For all age groups, whites reported higher rates of tobacco, alcohol and lifetime illicit drug use than did blacks and Mexican-Americans. Whites (7%) and Mexican-Americans (5%) < 18 years of age were more likely to report illicit drug use during pregnancy than were white (4%) or Mexican-American (2%) patients aged 18-21 and white (4%) or Mexican-American (0%) patients > 21 years. Blacks > 21 years of age reported the highest rate of current illicit drug use overall (9%). The rates of substance use reported by Mexican-Americans differed according to their spoken language, with English-speaking Mexican-Americans of all ages more likely to report tobacco, alcohol or illicit drug use than were corresponding Spanish-speaking Mexican-Americans. Awareness of the racial/ethnic and age differences in the rates of tobacco, alcohol and illicit drug use will assist physicians in the early identification of and intervention with pregnant women who place themselves and their fetuses at increased risk of injury from substance use.