Myelomeningocele is a birth defect in which the backbone and spinal canal do not close before birth. The condition is a type of spina bifida.

Normally, during the first month of a pregnancy, the two sides of the spine (or backbone) join together to cover the spinal cord, spinal nerves and meninges (the tissues covering the spinal cord). Spina bifida refers to any birth defect involving incomplete closure of the spine.

Myelomeningocele is the most common type of spina bifida. It is a neural tube defect in which the bones of the spine do not completely form, resulting in an incomplete spinal canal. This causes the spinal cord and meninges (the tissues covering the spinal cord) to stick out of the child's back.

Myelomeningocele may affect as many as 1 out of every 800 infants.

The rest of myelomeningocele cases are most commonly:

• Spina bifida occulta, a condition in which the bones of the spine do not close but the spinal cord and meninges remain in place and skin usually covers the defect
• Meningoceles, a condition where the tissue covering the spinal cord sticks out of the spinal defect but the spinal cord remains in place.

Other congenital disorders or birth defects may also be present in a child with myelomeningocele. Hydrocephalus may affect as many as 90% of children with myelomeningocele. Other disorders of the spinal cord or musculoskeletal system may be seen, including syringomyelia and hip dislocation.

The cause of myelomeningocele is unknown. However, low levels of folic acid in a woman's body before and during early pregnancy is thought to play a part in this type of birth defect. The vitamin folic acid (or folate) is important for brain and spinal cord development.

Also, if a child is born with myelomeningocele, future children in that family have a higher risk than the general population. However, in many cases, there is no family connection.

Some theorize that a virus make play a role, since there is a higher rate of this condition in children born in the early winter months. Research also indicates possible environmental factors such as radiation.