Assessment of the fetus in a high-risk pregnancy uses a variety of tests for screening and continued detection of in utero compromise. This paper describes Foetos, an expert system designed to help clinical personnel to interpret several fetal assessment tests: fetal biophysical profile, contraction stress test, and nonstress tests. Foetos has been built using the knowledge engineering tool Genie, which adopts a mixed frame- and rule-based approach to represent the clinical knowledge in the field. Foetos includes diagnostic, prognostic, and therapeutic structures, based on heuristic interpretation of such tests and contextual structures which relate interpretation to the overall clinical picture. The results of initial retrospective and prospective program validation are included in the report. These results show a substantial level of agreement between Foetos' recommendations and clinical management. Interpretation of the results indicates that the obstetrical field could be an area of interest for the application of AI techniques.