Spina bifida: A major birth defect and a type of neural tube defect that involves an opening in the vertebral column caused by the failure of the neural tube to close properly during embryonic development. (The neural tube is the structure in the developing embryo that gives rise to the brain and spinal cord.) Because of the defect in the spine, part of the spinal cord is exposed and protrudes as a meningomyelocele. People with spina bifida often have neurological deficits below the level of the lesion and can suffer from bladder and bowel incontinence, limited mobility (due to paralysis of the legs), and learning problems. The risk of spina bifida varies according to country, ethnic group, and socioeconomic status. In the US as a whole, spina bifida occurs in 1 in every 1,000 to 2,000 births. The risk of spina bifida and other neural tube defects, such as anencephaly, can be significantly decreased if women take ample folic acid before conception and during pregnancy.