Reporting Down syndrome screening results: women's understanding of risk.

Objectives: To compare women's understanding of different methods of expressing Down syndrome risks.

Methods: A self-administered structured questionnaire given to 311 English-speaking women postpartum, at three maternity units. Understanding of numeric risk expression was assessed by women identifying whether a specified risk was higher, lower or the same as another nominated risk, expressed as two percentages, as two ratios or one of each. Perceptions of a high-risk result were obtained using display rankings of percentages and ratios.

Results: Response rate was 95% (294/311). Overall, women were poor comparing numeric risks whether expressed similarly (ratio vs. ratio or percentage vs. percentage) or not. When comparing similarly expressed risks, 66% (95% CI: 62-70%) of respondents were correct, considerably more than when asked to compare different risk expressions 30% (95% CI: 26-34%), P < 0.0001. Women were more tolerant of risk when expressed as a percentage than as a ratio (median high risk for percentage form was 5% (1:20) and for ratio form was 1:200 (0.5%).

Conclusions: Women's understanding of Down syndrome risk is dependent upon how risks are expressed. These findings may usefully direct how risk should be reported to women having prenatal screening for Down syndrome.