Coronal biparietal diameter. A reliable alternative to the traditional transverse biparietal diameter

The traditional ultrasonically derived fetal biparietal diameter (BPD) is important in the determination of fetal age, weight, and growth. This measure cannot be obtained, however, with direct occiput anterior or posterior or with a head in the pelvis. This study looked at the BPD utilizing coronal (anatomically at right angles to the traditional BPD through the thalami, third ventricle, and cavum septum pellucidum) rather than transverse section through the fetal head. In 265 patients at 14 to 41 weeks' gestation, simultaneous measurement of the coronal and transverse BPDs revealed that they were interchangeable (r = 0.99). The major drawback of the coronal BPD is the inability to obtain a head circumference because of the absence of the occipitofrontal diameter.