Spina bifida is a condition that occurs when a baby's spinal cord does not close completely during formation of the spine, states the Spina Bifida Association. It is the most common birth defect in the United States, as of March 2015.
There are four types of spina bifida: spina bifida occulta, spina bifida dysraphism, meningocele and spina bifida cystica, also known as myelomeningocele, according to the Spina Bifida Association.
Occulta, which is the least severe form, occurs in 10 to 20 percent of the population diagnosed with spina bifida, states the National Institute of Neurological Disorders and Stroke. This form rarely causes a disability. Myelomeningocele, the most severe form, occurs when components of the spine are exposed, often resulting in paralysis in the body below the opening in the spine.