Neural tube defect: A major birth defect caused by abnormal development of the neural tube, the structure that is present during embryonic life that gives rise to the central nervous system. Abbreviated NTD. NTDs are among the most common birth defects resulting in infant death and serious disability. There are a number of different types of NTDs, including anencephaly, spina bifida, and encephalocele. In anencephaly there is absence of the cranial vault (the skull) and absence of most or all of the cerebral hemispheres of the brain. Encephalocele is a hernia of part of the brain, and the membranes covering it (meninges), through a skull defect. Spina bifida is an opening in the vertebral column encasing the spinal cord. Through this opening, the spinal cord and the meninges may herniate to create a meningomyelocele. All pregnancies are at risk for NTDs. Factors that increase the risk include a prior NTD in the family and type 1 diabetes in the mother. More than half of NTDs can be prevented if women consume supplements that contain folic acid before and during the early weeks of pregnancy in addition to getting folate in their diets. Because the risk for NTDs is not totally eliminated by folic acid use, routine prenatal screening for NTDs is still advisable.