The AIUM Practice Guideline for the Performance of Obstetric Ultrasound Examinations recommends that the biparietal diameter/head circumference (BPD/HC) measurement be taken at the level of the thalami and cavum septi pellucidi (CSP).
Which image below shows the CSP?
The CSP is seen on a sonogram as an empty box (image on the right), and the columns of the fornix have a line going through the box (image on the left). Although the columns of the fornix and thalami are still accurate levels for the BPD/HC measurement, you lose the diagnostic information that a normal CSP would demonstrate. With a normal CSP a midline brain defect can be ruled out.