Multiple simultaneous predictors of gestational age. An application of Bayes' theorem.

Most obstetricians share the belief that an estimate of gestational age based on several clinical measures that agree with one another is more accurate than an estimate based on a single measurement. This paper presents a formal justification for this belief by applying Bayes' theorem to calculate the probability of a fetus being a certain gestational age by using information from several clinically established single predictors. The application of Bayes' theorem justifies the use of multiple measures of gestational age in individual ultrasound examinations and the use of serial ultrasound studies in the third trimester to more accurately estimate gestational age. The same technique could be applied to obstetric research studies that need to establish accurate gestational dating.