Nomograms to help inform women considering Down's syndrome screening.

Objective: To derive graphical information for use in counselling women considering whether or not to have maternal serum screening for Down's syndrome.

Design: Statistical modelling of the frequency distribution of estimated Down's syndrome risk for four marker combinations.

Results: Nomograms are provided showing for each maternal age: (a) the detection and false-positive rates, and (b) the proportion of pregnancies with different estimated risks.

Conclusion: When screening is offered, clinicians need to have information readily available on test accuracy and the likely result, which is specific to the individual.