This study evaluates a scale measuring knowledge about a screening test and investigates the association between knowledge, uptake and attitudes towards screening. One thousand four hundred ninety-nine pregnant women completed the knowledge scale of the multidimensional measure of informed choice (MMIC). Three hundred forty-five of these women and 152 professionals providing antenatal care also rated the importance of the knowledge items. Item characteristic curves show that, with one exception, the knowledge items reflect a spread of difficulty and are able to discriminate between people. All items were seen as essential or helpful by both women and health professionals, with two items seen as particularly important and one as unimportant. There were some differences between health professionals, women with low risk results and women with high risk results. Knowledge was not associated with uptake, attitude, or the extent to which uptake was consistent with women's attitudes towards undergoing the test.