This report discusses the role of prenatal screening in preventing congenital abnormalities or, when prevention is not possible, in avoiding the conception or the birth of those who would have untreatable abnormalities. Women who are found by screening not to be immune to rubella can be safely vaccinated prior to pregnancy; those found to be at risk of having children with genetic disorders such as Tay-Sachs disease or thalassemia have the option of avoiding the conception of affected offspring. Screening during pregnancy permits the primary prevention of Rh disease and its sequelae when it results in the prophylactic administration of Rh-immune globulin to unsensitized Rh-negative women. Maternal serum alpha-fetoprotein screening identifies pregnant women who are at increased risk of carrying fetuses with neural tube defects or Down's syndrome, giving them the option of avoiding the birth of affected fetuses through abortion. Recombinant DNA technology will permit screening for many more genetic disorders as the disease-related genes and mutations are identified. For many of these disorders, the ability to predict the risk of disease will antedate preventive and therapeutic interventions by many years. During this lag phase, issues concerning the validity of the tests, the severity of the conditions for which screening is offered, the safety of the interventions, and the autonomy of the pregnant women in deciding to be screened are important.