Decision aids help patients make treatment choices. There is little empirical evidence to explain how they work. The results from this randomised controlled trial comparing routine with decision-aided consultations in the prenatal diagnosis for Down's syndrome context are used to describe the strategies employed during decision making, to assess the impact of a decision aid on decision processes, and to investigate decision process and outcome associations. Data were elicited from two content analyses of consultation transcripts and questionnaires assessing knowledge, anxiety, decisional conflict, reasons, and information usefulness. 68/106 women completed measures at consultation and follow-up. Decision-aided women employed more cognitive and emotional strategies during decision making. More negative evaluations during decision making were associated with better outcomes. Decision-aided consultations facilitated the employment of strategies associated with more effective choices. These consultations take longer and elicit greater expressions of negative affect, so may be less rewarding encounters for health professionals.