Spina bifida is a neural tube defect that is characterized by the incomplete development of the brain, spinal cord, and/or their protective coverings. This condition occurs when a fetus's spine fails to close properly during its first month of development. The three most common types of spina bifida are myelomeningocele, meningocele, and spina bifida occulta. Spina bifida is the most common neural tube defect in the United States, affecting 1,500 to 2,000 babies each year.