INTRODUCTION: In Stockholm County, new guidelines for predicting the day of delivery were introduced in 2010. Recent clinical use has indicated that the predictive quality might be suboptimal. This study compares the accuracy of three equations applied during the first (crown-rump length or bi-parietal diameter) and second trimesters (bi-parietal diameter or bi-parietal diameter combined with femur length). MATERIAL AND METHODS: The accuracy of estimated delivery date was compared in 14 239 ultrasound exams using median deviations from actual birth date, proportion of postterms, births within ±seven days of estimated delivery date, accuracy variations depending on fetal size, alternative fetal measurements obtained on the same occasion, and menstrual age. RESULTS: The bi-parietal diameter and crown-rump length formulae overestimated pregnancy length by two and three days respectively, causing 7-8% of pregnancies to be labelled postterm. A combined bi-parietal diameter+femur length formula overestimated by one day, with 5.1% postterms. No significant difference was found in the proportion of births within ±seven days. Second trimester estimated delivery date assessment had larger median variations than did first trimester assessment and suffered from shifting deviations across fetal size. The comparison of different biometry formulae in the same individual demonstrated one day extra deviation for bi-parietal diameter and three days extra deviation for crown-rump length compared with the combined bi-parietal diameter+femur length formula. CONCLUSIONS: The algorithms and dating occasions tested seem inappropriate for the present 280-day term definition. Alternative formulae ought to be sought, and the assumed duration of pregnancy reconsidered; 283 days corresponds to the observed pregnancy length calculated from last menstrual period, and would better fit the observed results for first trimester ultrasound scans.