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.