Myelomeningocele is the most severe form of spina bifida. It happens when parts of the spinal cord and nerves come through the open part of the spine. It causes nerve damage and other disabilities. Seventy to ninety percent of children with this condition also have too much fluid on their brains (hydrocephalus). This happens because fluid that protects the brain and spinal cord is unable to drain like it should. The fluid builds up, causing pressure and swelling. Without treatment, a persons head grows too big, and theymay have brain damage. Other disorders of the spinal cord may be seen, including syringomyelia and hip dislocation. The cause of myelomeningocele is unknown. However, low levels of folic acid in a woman's body before and during early pregnancy is thought to play a part in this type of birth defect. The disease is myelomeningocele (MONDO_0019773).