Although most women report alcohol use, women generally are light drinkers. Those who drink and drink heavily are more likely to be young, white, single, to have a higher education and income, and to be employed outside the home. However, women who drink during pregnancy, and particularly, those who continue to drink through the third trimester are different. They are older, more likely to be black, and they have higher rates of illicit drug use, less education, and lower social status. Marijuana and cocaine are used less frequently. However, women of childbearing age have the highest rates of use for both these drugs. Women who use marijuana during pregnancy are more often black, unmarried, and of lower social class. Cocaine users tend to be black, older, unmarried, and also of lower socioeconomic status. Both groups more frequently use other illicit drugs and, in general, receive less prenatal care. Therefore, for alcohol, marijuana, and cocaine, the highest rates of use are found among women of childbearing age. The women most likely to use substances during pregnancy are women who also have other characteristics that are, in themselves, significant risk factors for poor pregnancy outcome. These covariates must be considered in the evaluation of the effects of prenatal substance use.