Data on 100 consecutive patients delivering within 1 week after ultrasonic cephalometry measurement were reviewed to assess the weight-predicting potential of three known formulas. With Thompson's formula, 78% of infant weights fell within a range of 350 g, the absolute mean error for the 100 cases being 267 g. Excluding cases of fetal malnutrition, when the biparietal diameter (BPD) obtained was 8.3 cm or more, all newborn babies weighed more than 2000 g, and with a BPD of 8.7 cm or more they all exceeded 2500 g. Ninety percent of newborn babies in whom the in utero BPD was 9.1 cm or more weighed more than 3000 g. These values compared very favorably with those from other series and, thus, demonstrate the usefulness of this type of assessment.