What are the social consequences of the recent expansion of newborn screening in the United States? The adoption of new screening technologies has generated diagnostic uncertainty about the nature of screening targets, making it unclear not only whether a newborn will develop a disease but also what the condition actually is. Based on observations in a genetics clinic and in-depth interviews with parents and geneticists, we examine how parents and clinical staff work out the social significance of uncertain newborn screening results. We find that some newborns will experience a specific trajectory of prolonged liminality between a state of normal health and pathology. Based on a review of related literatures, we suggest “patients-in-waiting” as an umbrella concept for those under medical surveillance between health and disease.