Comparison of ultrasonic and direct biparietal diameter measurements and placental localization

The accuracy of fetal biparietal diameter (BPD) measurements and placental localizations performed by physicians in a university hospital obstetrical service using real-time ultrasound equipment was determined. The sonographic estimations were compared to caliper measurements on the neonate and to the implantation site of the placenta found at surgery in 40 patients undergoing repeat cesarean section. The correlation coefficient 0.93 and p less than 0.00001 indicate high agreement between the BPD measurements. The regression of caliper on sonar measurements gave 95% prediction intervals of +/- 3.50 mm compared to +/- 2.4 mm calculated from data reported by ultrasonic laboratories. Placental localization was in error in 5.9% of the cases. Thus, physicians should be aware that their errors are likely to be larger than those which have been published by experts in ultrasound diagnosis.