Medical screening programs have been established based on the idea of early treatment and prevention. The selection of tests as well as the diseases tested for that are included in a screening need to be ethically justified. This article looks at criteria for screening such as severity of a disease and efficiency of a test and their applicability for newly developed preconceptional genetic carrier screening. Preconceptional genetic carrier screening can be used by couples planning a pregnancy in order to learn (...) it they are both carriers of a recessive inherited diseases und thus have an increased risk of having an affected child. It is argued that introducing fuzzy logic helps to overcome a binary logic of ethical decisions and to discuss ethical problems connected to the selection of diseases for preconceptional genetic carrier screening in a more nuanced way. With ongoing research about the relationship between genotype and phenotype as well as current development of new genetic screening methods the selection of tests and diseases will have to deal with knowledge deficits which fuzzy logic can help to handle. (shrink)