Spina bifida is a birth defect that occurs when the spine and spinal cord don't form properly. It falls under the broader category of neural tube defects. The neural tube is the embryonic structure that eventually develops into the baby's brain and spinal cord and the tissues that enclose them.
Normally, the neural tube forms early in pregnancy, and it closes by the 28th day after conception. In babies with spina bifida, a portion of the neural tube fails to develop or close properly, causing defects in the spinal cord and in the bones of the spine.
Spina bifida can range from mild to severe, depending on the type of defect, size, location and complications. When early treatment for spina bifida is necessary, it's done surgically, although such treatment doesn't always completely resolve the problem.