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Spina bifida. Spina bifida is a birth defect involving  incomplete closure of the spinal column during fetal development. In the most severe cases, part of the spinal cord protrudes from the infant's back, covered only by a thin sac. Doctors may perform corrective surgery on the infant during pregnancy by making a small incision in the mother's belly, then carefully cutting into the uterus. The spinal cord is maneuvered into place and the hole closed up.