Before 12-16 weeks of pregnancy, each of the substances was used by over 50 percent of this population, with 53 percent smoking cigarettes, 51 percent smoking marijuana, and 58 percent consuming beer or wine. Most substance use ceased after 12-16 weeks of pregnancy. There were no significant differences between ethnic groups in substance use. Multiple regression analyses showed that cigarette smoking decreased when social support was provided by the adolescent's partner and it was higher if parents smoked and/or used alcohol (p less than .001). Parental substance use, combined with lack of social support, was also associated significantly with marijuana use (p less than .001), explaining 30 percent of the variance. Use of birth control prior to pregnancy was related to beer and wine consumption. (Author abstract)