A prerequisite to an informed decision to undergo any screening or diagnostic test is knowledge about such a test. This study describes the development of a self-administered questionnaire to measure knowledge of prenatal tests, for use in studies concerning the uptake of these tests. Both the reliability (internal and test-retest) and predictive validity are evaluated. Validity is assessed in two studies: first, comparison is made between three criterion groups (two with experience of pregnancy, one without); and second, scores on the questionnaire are compared before and after women have been provided with information about possible tests. The results show this to be both a reliable and valid measure of knowledge about prenatal tests. The results also highlight aspects of knowledge that are lacking. For example, a sizeable minority of women were uncertain or incorrect in identifying which tests they had undergone in a recent pregnancy.