Spina bifida is a birth defect that occurs when the tissue surrounding the developing spinal cord of a fetus does not form properly. This defect can occur anywhere along the spine but is most often found in the lower spine. Spina bifida is the most common group of birth defects called 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.

Spina bifida develops in a fetus early in pregnancy. Typically, the neural tube forms in the developing baby early in the pregnancy and closes by the 26th day after conception. Later, the top of this tube becomes the baby's brain, and the remainder of the tube becomes the baby's spinal cord. 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 that make up the backbone. In the United States, approximately 1 in every 2,000 children are born with spina bifida.