The monitoring of fetal weight is an important aspect of antenatal care. To construct an individually adjustable standard, we developed a model to link the predicted birth weight to a fetal weight curve which outlines how this weight is to be reached in an uncomplicated pregnancy. A formula was derived which describes the median fetal weight at each gestation as a proportion of the optimal term weight, and also defines the 90th and 10th centile curves as normal limits. We analyzed a birth weight database of 38,114 singleton, routine ultrasound-dated pregnancies resulting in term deliveries. By stepwise multiple regression analysis, we derived coefficients for the factors that act as variables on term birth weight in our population. Apart from gestational age and sex, the maternal height, weight at first visit, ethnic group, parity and smoking all have significant and independent effects on birth weight. The variation due to ethnic group appears to be physiological in this population. Smoking is associated with a reduction in birth weight, which is independent of maternal physique and related to the number of cigarettes per day as reported at the first visit. We have developed a software program which calculates, on the basis of pregnancy variables entered at the first visit, an adjusted normal range for fetal size. This can be printed out as a chart and used for antenatal surveillance of growth.