David J.R. Hutchon, MD

Many clinicians and perhaps some statisticians are at odds regarding the correct application of Bayes theorem in integrated risk assessments of screening programs for Down syndrome1. Most standard textbooks show that the posterior odds = prior odds X likelihood ratio but some publications show the use of prior risk X likelihood ratio to calculate the posterior risk. Bayes theorem does refer to probabilities, which is equivalent to the word "risk"

The object of this paper is to discuss the routine use of ultrasound in estimating the date of delivery and to demonstrate an approach which has both sound epidemiological and statistical bases. CESDI (Confidential Enquiry into Still Births and Deaths in Infancy) criteria for determining gestation, based on menstrual dates and cycle length superseded by an ultrasound scan different by more than ten days, are difficult to apply.