How do anencephaly microcephaly and hydrocephalus differ as congenital nervous-system disorders?

Anencephaly, microcephaly, and hydrocephalus are distinct cephalic disorders that affect the development and structure of the brain and skull in different ways:

• Anencephaly: This condition involves the absence of significant portions of the brain and skull. It results from a failure of the upper neural tube to close during the first month of embryonic development. It is a severe defect that is typically fatal at or soon after birth.1
• Microcephaly: Meaning "small head," this disorder is defined by a head circumference significantly smaller than average for an individual's age and sex. While often associated with a diminished brain size, the brain may otherwise be normal in structure. It can be caused by genetic factors, maternal infections, or environmental exposures during pregnancy.12
• Hydrocephalus: This is characterized by the accumulation of cerebrospinal fluid within the brain's ventricles. Unlike the reduced size seen in microcephaly, the pressure from this fluid buildup causes the head to enlarge. It is often caused by obstructions in fluid drainage and may require surgical intervention, such as a shunt, to manage pressure.3