Substance Abuse and Pregnancy. Alcohol, cigarettes, and street drugs (cocaine, marijuana, and heroin) are known to have harmful effects on the fetus. Any amount of these substances is considered unsafe during pregnancy. The best goal is to avoid them altogether.rug use early during pregnancy can affect the developing organs and limbs of the fetus. Even one episode of drug use during this period can affect the development of your child. Often the result is a birth defect or miscarriage.