People who are born with hydrocephalus have a type of hydrocephalus called congenital hydrocephalus. Congenital hydrocephalus is usually caused by a birth defect or by the brain developing in such a way that the cerebrospinal fluid (CSF) in the brain cannot drain properly. Most cases of hydrocephalus (more than 70%) occur during pregnancy, at birth, or shortly after birth. Causes of congenital hydrocephalus include:

• Toxoplasmosis (an infection from eating undercooked meat, or by coming in contact with infected soil or an infected animal)
• Cytomegalovirus (CMV, infection by a type of herpes virus)
• Rubella (German measles)
• A genetic disorder usually passed only from mother to son

 Hydrocephalus can also develop later in life. This type of hydrocephalus is called acquired hydrocephalus, and it can occur when something happens to prevent the CSF in the brain from draining properly. Causes of acquired hydrocephalus include:

• Blocked CSF flow
• Brain tumor or cyst
• Bleeding inside the brain
• Head trauma
• Infection (such as meningitis)