On the basis of the hypothesis that undisturbed individual growth in fetal life keeps a constant proportional difference with the standard population 50th percentile, birth weight can be predicted with a single sonographic exploration after the 16th week of pregnancy. Data on 135 singleton pregnancies with accurate dates and delivery at term were used. Sonography was performed between the 17th and 36th weeks of pregnancy, in every case at least 4 weeks before delivery. The observed measurements of BPD, FL, and AC were used for the prediction of their values on the day of delivery, applying the Hadlock equation for the estimation of birth weight. The mean error of birth weight predictions was -1 +/- 11% (SD), with a correlation coefficient between observed and predicted birth weights of 0.62 (P < 0.001). The accuracy was not influenced by the gestational age at the time of exploration. The model underpredicts birth weight of larger fetuses while overpredicting that of lighter ones, but a part of the error could be explained by a change in the growth pattern after modeling. Regardless of the classification considered (i.e., gestational age at sonography or birth weight), over 83% of predictions had an error below 15%. The underlying hypothesis could be useful clinically in modeling and monitoring fetal growth, allowing application of the results of remote sonographic explorations in clinical management. In addition, being able to project fetal weight to the 40th week also is valuable in improving the patient's understanding of fetal growth.