22-week fetuses with hindbrain herniation

(A) 22-week male fetus. Longitudinal ultrasound image of the craniocervical junction shows crowding of the posterior fossa (arrow) with extension of hindbrain structures (arrowhead) into the upper cervical spinal canal posterior to the cord, compatible with hindbrain herniation and Chiari II malformation. (B) 22-week female fetus with a normal posterior fossa and craniocervical junction with open CSF spaces (dashed arrow), for comparison. (C) Same male fetus as in (A), with sagittal SSFSE T2-weighted MRI angled to simulate the ultrasound image showing crowding of the posterior fossa with effacement of the cisterna magna (thick arrow) and fourth ventricle (curved arrow), and downward projection of the hindbrain (arrowhead) into the upper cervical canal.