Fetal biparietal diameter: rational choice of plane of section for sonographic measurement.

In this sonographic study of 100 fetuses (gestational age 15--40 weeks) the maximum transverse diameter of the fetal skull at the axial plane described by Campbell for measurement of the frontooccipital head circumference was compared to similar measurements at specific anatomic planes above and below this level. Transverse diameter measurements at the head circumference level averaged 3 mm (range, 0--9 mm) more than at the level of the superolateral margins of the bodies of the lateral ventricles (where the midline echo is continuous), and 4 mm (range, 0--10 mm) more than at the level of the cerebral peduncles. The head circumference plane is the most appropriate level for measurement of the fetal bipariental diameter (BPD) because (1) the transverse diameter is maximal, (2) unlike the conventional BPD image, this section contains specific anatomic landmarks which should make the BPD measurements technically easier and more reproducible, and (3) stimultaneous measurements of the BPD, frontooccipital head circumference, occipitofrontal diameter, and cephalic index can be obtained.