What is a fetal arachnoid cyst?

A fetal arachnoid cyst is a sac filled with cerebrospinal fluid (CSF) that is located between the brain or spinal cord and the arachnoid membrane, which is one of the three membranes that cover the brain and spinal cord. Primary arachnoid cysts are congenital (present at birth), resulting from abnormal development of the brain and spinal cord during early pregnancy. They are usually an isolated finding but occasionally can occur as part of a wider genetic syndrome. The fetal medicine specialist will study the cyst with ultrasound regularly during the pregnancy, and may recommend MRI scan to assess the cyst further. Although many do not cause any problem during pregnancy, some can get larger and cause a pressure effect in the brain which may include a build up of fluid in the brain. Larger cysts will require support from paediatric neurology. More information can be found here.