Ultrasound is used to assess foetal age, foetal weight and growth. The error of such measurements is considerable, but the technique of averaging repeat measurements restricts random error. The use of customised foetal weight charts, that is, adjusting for ethnicity and maternal and foetal factors helps in classifying foetal weight appropriately. Commonly used cross-sectional reference ranges are useful for the foetal weight assessment at any stage of pregnancy, but not for foetal growth. Growth assessment requires serial measurements and longitudinal reference ranges, which provide conditional terms for individual foetuses. That is, an initial measurement is used for calculating individual ranges for the rest of pregnancy. Compared to the ranges for the entire population, the conditional ranges for a small foetus are narrower and skewed in the direction of the initial measurement. Quality control is recommended to ensure that such methods work when applied to the local population.