We describe new charts and tables for dating pregnancies derived from data collected in a study designed to enable the construction of these and charts of fetal size. This was a prospective study in which 663 fetuses seen in the routine ultrasound clinic in a London teaching hospital were scanned once only for the purpose of this study. We discuss the statistical methodology in detail and compare the new charts with other published charts. We suggest that the differences seen may be due to variation in study methodology.