A standardized plane for biparietal diameter measurement.

Biparietal diameter measurement is the most widely accepted means of determining gestational age. However, a standard anatomic plane for performing this measurement has not been established. Three hundred forty-four measurements on 256 patients who gave birth to normal infants were obtained and compared at various cranial levels based on brain anatomy. The results document that use of a standardized plane increases measurement reliability but does not significantly alter variability. We postulate that variability is based predominantly on biologic variation in fetal growth rates rather than on technical errors.