||This article aims at analyzing the factors that mold the material cultures of contemporary biomedicine. It considers the historical development of diagnostic techniques, how they define the "norm", and influence the evolution of conducts of professionals as well as those of concerned members of the family. Following a brief reconstruction of the historical evolution of pre-natal diagnostics, we present a detailed case analysis of a particular anomaly: aneuploidy of the sexual chromosomes (ACS). Although some cases of ACS may represent grave and even life-threatening deficiencies, the great majority of children who possess an abnormal number of sexual chromosomes experience relatively minor problems. In many cases, the very diagnostic of ACS only surfaces during the person's adolescence. Thus, especially in contexts that permit legal abortion, the prenatal diagnosis of these syndromes renders visible the construction of the "abnormal fetus" and the "risk of having an abnormal child", as a techno-social phenomenon that emerges over time through the interaction of biomedical techniques, the organization of medical work, legal limitations, and sociocultural considerations.