Spina bifida is one of a group of disorders called neural tube defects — malformations of the brain, spinal cord or their coverings. Spina bifida occurs when a fetus's developing spinal column does not close properly in the first 28 days following fertilization. The disorder can take one of three forms:
- Spina bifida occulta — The spinal column is not completely closed. This may not cause any symptoms and often requires no treatment.
- Meningocele — Part of covering of the spinal cord called the meninges may protrude through an opening in the back.
- Myelomeningocele — A portion of the spinal cord itself protrudes through an opening in the back.
Meningoceles and myelomeningoceles may appear as saclike structures on an infant's back at birth. When most people refer to spina bifida, they are referring to the most severe form, myelomeningocele. The incidence of neural tube defects varies by country, ranging between 1 and 10 per 10,000 births. Higher rates are often reported among poorer populations.