Problems and pitfalls of risk assessment in antenatal care.

Current obstetric risk-scoring systems do not make a precise prediction of the chances of an abnormal outcome and so cannot be used in formal decision analysis. We examine here the feasibility of using Bayes' theorem to provide an accurate assessment of fetal risk and conclude that two severe limitations effectively exclude this approach as a useful contribution to antenatal care: (1) inaccuracy of the data base: geographical variations and the 'treatment paradox' conspire to reduce the reliability on which any assessment may be made; (2) 'dependency' of the risk factors: most obstetric variables are interdependent and are not therefore amenable to analysis by means of Bayes' theorem. Although fetal risk will be assessed subjectively for the foreseeable future, obstetricians should be aware of the essentially mathematical nature of decision-making.