Measurements of the biparietal and anteroposterior chest diameters were made on 1079 newborn infants. These measurements were compared with fetal weight and gestational age. Estimates of birth weight from the biparietal diameter alone had a prediction error standard deviation of ± 350.36 g. When the anteroposterior chest diameter was used the prediction error standard deviation was ± 364.14 g. When both of these diameters were used, the prediction error standard deviation was 289.73 g. A similar comparison with gestational age, using both of these dimensions, was not significantly better than predicting gestational age from the last menstrual period. Formulas and graphs to predict fetal weight and gestational age from these measurements are presented.