Despite considerable concern of bioethicists, disabilities rights activists, feminists and others about the spread of prenatal diagnostic technologies, their routine acceptance in many parts of the world continues at a rapid pace. Yet, there is wide variation by country and region in rates of acceptance of prenatal diagnosis. We draw on John McKinlay's model of how a medical innovation becomes routinized to explore the circumstances that led to the widespread use of one prenatal diagnostic screen-the maternal serum alpha fetoprotein (MSAFP) test for the detection of neural tube defects and other developmental disabilities. As predicted by McKinlay's model, analysis of published data suggests that strong institutional or provider support is the best predictor of women's level of MSAFP test acceptance. Data collected at a health maintenance organization in California illuminate the processes through which medico-legal and institutional forces affect the use of MSAFP screening. By examining the language women use to talk about MSAFP screening, we show how providers also shape women's understandings of the meaning and purpose of MSAFP screening. These data ultimately shed light on how the very ethical issues which concern critics of prenatal diagnosis become obscured in the processes by which this screening test becomes accepted as routine.