Hydrocephalus is not itself a neural tube defect, but the two are closely linked. Neural tube defects are a specific group of birth defects that occur when the neural tube fails to close properly during early embryonic development. Hydrocephalus, the buildup of fluid in the brain, is most often a consequence of those defects rather than one of them.
What Counts as a Neural Tube Defect
The neural tube is a structure that forms in the embryo between 17 and 30 days after conception. It eventually becomes the brain and spinal cord. When part of the tube doesn’t close completely, the result is a neural tube defect. The CDC recognizes five primary types:
- Anencephaly: partial or total absence of the brain and skull
- Spina bifida: incomplete closure of the vertebrae, with or without exposed spinal tissue
- Encephalocele: brain tissue herniating through a gap in the skull
- Craniorachischisis: anencephaly combined with an open spinal defect
- Iniencephaly: a defect in the back of the skull with severe backward bending of the head and trunk
Hydrocephalus does not appear on that list. It is classified separately in medical coding systems. In ICD-10 and ICD-11, hydrocephalus has its own diagnostic codes, distinct from the codes used for spina bifida, anencephaly, and other neural tube defects.
Why Hydrocephalus and Neural Tube Defects Overlap So Often
Although hydrocephalus is not a neural tube defect by definition, it is the single most common condition associated with them. About 80% of infants born with myelomeningocele, the most severe form of spina bifida, develop hydrocephalus. Data from the National Spina Bifida Patient Registry confirms this: of more than 4,400 patients with myelomeningocele tracked across 26 institutions, roughly 80% underwent at least one surgical procedure to treat hydrocephalus.
The connection is mechanical. In most cases of spina bifida, the base of the brain doesn’t develop correctly. The brain stem and lower part of the cerebellum get pushed downward into the spinal canal, a condition called a Chiari II malformation. That displacement blocks the normal flow of cerebrospinal fluid (the liquid that cushions the brain). When fluid can’t drain properly, it accumulates inside the skull, enlarging the ventricles of the brain. This is obstructive hydrocephalus, and it is a direct downstream effect of the neural tube defect, not a separate defect in the tube itself.
Hydrocephalus Without a Neural Tube Defect
Plenty of babies are born with hydrocephalus that has nothing to do with the neural tube. A narrowed or blocked cerebral aqueduct, the tiny channel connecting the brain’s ventricles, is one of the most common causes. Other causes include Dandy-Walker malformation (a developmental problem in the back of the brain), premature birth, bleeding inside the brain’s ventricles, infections during pregnancy, and genetic conditions. Some of these causes are structural, some are acquired, and some remain unexplained.
This is a key distinction. When hydrocephalus appears alongside spina bifida or encephalocele, it is a complication of a neural tube defect. When it appears on its own, it is a separate condition with its own set of causes.
What This Means for Prevention
Folic acid supplementation before and during early pregnancy is one of the most effective ways to prevent neural tube defects. Because hydrocephalus so often results from those defects, folic acid indirectly reduces hydrocephalus cases too. But a large study from China found something more surprising: folic acid taken around conception also reduced the risk of congenital hydrocephalus that was not linked to a neural tube defect. Women with high adherence to supplementation saw roughly a 66% lower risk of non-neural-tube-defect hydrocephalus compared to those who didn’t supplement.
This doesn’t mean folic acid prevents all forms of hydrocephalus. Cases caused by prematurity, infection, or bleeding in the brain after birth are different problems entirely. But it does suggest that folic acid’s protective effects on early brain development extend beyond the neural tube itself.
The Short Answer
Hydrocephalus is not classified as a neural tube defect. It is a separate condition defined by excess cerebrospinal fluid in the brain. The reason the two are so frequently discussed together is that neural tube defects, especially spina bifida, cause hydrocephalus in the vast majority of cases. Think of it this way: a neural tube defect is the structural problem, and hydrocephalus is often what happens because of it.