A major birth defect caused by abnormal development of the neural tube, or the structure in an embryo which develops into the brain and spinal cord. Neural tube defects are among the most common birth defects that cause infant death and serious disability. The most common neural tube defects are anencephaly, spina bifida, and encephalocele. In anencephaly the skull and most or all of the brain does not develop. Encephalocele is a hernia of part of the brain and of the membranes covering it. Spina bifida is an opening in the column encasing the spinal cord.