Embryology · Nervous system and senses
Neural tube defects: anencephaly, spina bifida and encephalocele !
- Malformation
If the neural tube does not close in the first month, the brain or spinal cord is left exposed; folic acid prevents many of these defects.
Updated 2026-11-01
See it in 3D: Spinal cord · Conus medullaris · Dura (dural sac) · Occipital lobe
What it is
Neural tube defects (NTDs) result from incomplete closure of the neural tube in week 4, often before the pregnancy is known. Failure at the cranial end causes anencephaly; failure at the caudal end causes open spina bifida; bony skull defects through which meninges or brain protrude are encephaloceles. They are among the most common serious birth defects and among the most preventable.
Key facts
- Anterior neuropore: closes around day 25; posterior neuropore: around day 28.
- Anencephaly: cranial closure fails; the vault does not form and the exposed tissue degenerates. It is lethal.
- Spina bifida: almost always lumbosacral; ranges from the occult form to myelomeningocele.
- Encephalocele: most often occipital; can be frontonasal or parietal.
- Risk factors: low folate, poorly controlled maternal diabetes, obesity, fever or overheating, valproate and carbamazepine.
- Folic acid supplementation before conception and in the first trimester prevents a large share; in Colombia wheat flour is fortified with folic acid.
Embryological cause
- Week 4: the neural folds fail to fuse along one segment of the tube.
- Cranial failure (anterior neuropore) → exencephaly that degenerates into anencephaly; if it extends into the cord, craniorachischisis.
- Caudal failure (posterior neuropore) → myeloschisis or myelomeningocele, with open vertebral arches.
- In spina bifida occulta the tube closes but the vertebral arches fail to fuse (a sclerotome defect).
- Genes of the planar cell polarity pathway and folate metabolism contribute.
Types of neural tube defect
| Type | What protrudes | Skin | Maternal AFP | Outlook |
|---|---|---|---|---|
| Anencephaly | Exposed brain tissue, no vault | Absent | Very high | Lethal |
| Encephalocele | Meninges ± brain through skull | Usually covers | Normal or high | Depends on tissue herniated |
| Spina bifida occulta | Nothing; open arch only | Intact | Normal | Usually benign |
| Meningocele | Sac of meninges and CSF | Variable | High if open | Good, few deficits |
| Myelomeningocele | Meninges + cord and roots | Absent | High | Paraplegia, neurogenic bladder, Chiari II |
| Myeloschisis (rachischisis) | Flat exposed neural plate | Absent | High | Most severe |
Markers
- AFP
- acetylcholinesterase
- folate
- VANGL1
- MTHFR
How it presents
- Anencephaly: absent skull vault and polyhydramnios (the fetus does not swallow).
- Myelomeningocele: lumbosacral sac, flaccid paralysis and sensory loss below the lesion, clubfoot, neurogenic bladder and bowel.
- Most children with myelomeningocele have Chiari II and hydrocephalus needing a shunt.
- Spina bifida occulta: hair tuft, dimple or patch over L5–S1; sometimes tethered cord.
- Latex allergy is common in children who have had many operations.
Diagnosis
- Raised AFP in maternal serum (second-trimester screen) and in amniotic fluid, with acetylcholinesterase, in open defects.
- Ultrasound: absent skull vault (anencephaly, visible from the first trimester).
- Ultrasound in spina bifida: the 'lemon' sign (flattened forehead) and 'banana' sign (squeezed cerebellum).
- Fetal MRI for the level of the lesion; after birth, imaging of spine and brain.
Test yourself
Think of the answer, then tap the question to see it.
Which neuropore fails in anencephaly, and around which day does it normally close?
The anterior (cranial) neuropore, which closes around day 25.
Besides AFP, which amniotic fluid finding supports an open defect?
Raised acetylcholinesterase, which comes from the exposed neural tissue.
Why does anencephaly come with polyhydramnios?
Because the fetus lacks an effective swallowing reflex and does not swallow amniotic fluid.
In which form of spina bifida is AFP usually normal?
Spina bifida occulta, because skin covers the defect and no tissue is exposed.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Neurulation and closure of the neural tube
- Folic acid and neural tube defect prevention
- Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesis
- Chiari malformations
- Congenital hydrocephalus: aqueductal stenosis
- Spinal cord: alar and basal plates and the ascent of the cord
- Valproate, carbamazepine and phenytoin
- Maternal diabetes and obesity as teratogens
Related conditions
In Pharmacology
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: About Spina Bifida (Centers for Disease Control and Prevention, 2024); Anencephaly (Centers for Disease Control and Prevention, 2024); Encephalocele (Centers for Disease Control and Prevention, 2024); Langman's Medical Embryology, 13th ed., ch. 18: Central Nervous System (Wolters Kluwer, 2015).
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Representative model, for educational purposes only.