They originate during the earliest weeks of pregnancy. This is the least serious form of the defect. A birthmark or a patch of hair may appear in the skin directly over it, or a growth of fatty tissue may cause a small bulge or a dimple. Spina bifida occulta usually causes no immediate health problems. As an afflicted child grows, the tethered cord may become stretched, causing numbness in the legs or occasional loss of bladder control.
Meninges are three-layered membranes that surround the brain and spinal cord.
A meningocele occurs when the opening in one or more vertebras is extensive enough to allow the meninges to balloon through it. The bulge produces a noticeable lump under the skin - it may be as small as a grape or as large as a grapefruit. If it contains only cerebrospinal fluid, it will not cause immediate harm. But if any of the nerves directly communicating with the spinal cord protrude into the sac, muscle weakness and some loss of bladder control may result. Myelomeningocele is the most common as well as the most severe form of the defect.