The aim of this study was to compare the clinical performance of ultrasound dates and ultrasound dates combined with menstrual dates for the detection of post-maturity. A computer model was designed which uses the statistical distributions of the duration of normal pregnancy, day of ovulation in relation to the menstrual cycle and ultrasound error for estimating gestational age. The clinical performance of the different dating methods was then analysed from these variables, on simulations of 30,000 cases. The efficacy of different dating methods for detecting post-maturity was determined by generating receiver-operator characteristics (ROC) curves. The proportion of post-term pregnancies (294 days and over) predicted by the model (3.5%) agrees with published values. There is a steep rise in missed cases if induction is delayed beyond 10 days from the expected date of delivery, reaching 20% on day 294. Elective delivery on day 290 will detect 98.9% of cases destined to deliver post-term, with an induction rate of 10%; the respective figures for induction on day 294 are 79% and 3.8%. The ROC curves for the detection of post-maturity suggest that use of the mid-trimester biparietal diameter (BPD) is better than a 7-day or 10-day rule. Timing of elective delivery is the most important variable affecting the detection rate for post-maturity There is no advantage in using menstrual dates when ultrasound biometry is available.