The most common causes for an enlarged head are megalencephaly, or an enlarged brain, and hydrocephalus, or excessive cerebrospinal fluid (CSF) in the brain.
When macrocephaly is a result of megalencephaly, it is often impossible to determine the cause.
Excessive brain size is termed megalencephaly (literally "big brain").
Megalencephaly is defined as any brain size above the 98th percentile within the population.
Megalencephaly due to neurological or metabolic disease: Mental retardation, seizures.