The small anechoic CSF-filled box located in the midline brain at the level of the BPD is the?

• A. Cavum septum pellucidum
• B. Corpus callosum
• C. Thalamus
• D. Cavum vergae

Answer: (A)

On fetal brain ultrasound, the small midline, CSF-filled space seen between the leaves of the septum pellucidum is the cavum septum pellucidum. It sits along the midline, anterior to the third ventricle and just above the corpus callosum, and is most clearly seen on an axial view at roughly the level of the biparietal diameter. Because cerebrospinal fluid reflects as anechoic, this space appears as a tiny dark box between the echogenic septal leaflets. This is a normal finding in the mid-trimester and early third trimester, though the cavum septum pellucidum typically closes after birth. The cavum vergae is a posterior extension that would be seen behind the CSP, not as the small box at this level.