What is the importance of the fetal cisterna magna measurement during routine second trimester Ultrasound?
A normal fetal cisterna magna diameter is <10 mm (usually 2 to 8 mm). Effacement of the cisterna magna suggests a possible Chiari II malformation with a meningocele or myelomeningocele. Enlargement of the cisterna magna can be seen with a Dandy Walker malformation, in which case there is also cerebellar vermian hypoplasia or agenesis and communication of a posterior fossa cyst with the fourth ventricle. An enlarged cisterna magna with a normal appearing cerebellum and brain and no communication with the fourth ventricle can be a normal variant known as mega cisterna magna.
