Where should calipers be placed when measuring biparietal diameter (BPD)?

When measuring the biparietal diameter (BPD) in obstetric ultrasound, the calipers should be placed from the outer edge of one parietal bone to the outer edge of the opposite parietal bone, which is referred to as "outer to outer." This method provides an accurate measurement of the fetal head's transverse diameter and helps in determining gestational age and assessing fetal growth.

Correctly using the outer edges avoids including any soft tissue, such as the scalp, which could lead to an overestimation of the true BPD. The placement of calipers in this fashion ensures that the measurement reflects the bony structures of the fetal skull, which is crucial for reliable assessment during prenatal evaluations.