operation done for that purpose); 20 percent are using highly effective contraceptives, usually long-term in nature; 5 percent are pregnant or immediately postpartum at any particular point in time; and 11 percent have never had sexual intercourse.
NTDs are the most common major congenital malformations of the central nervous system. They arise as a result of a disturbance of the embryonic process of neurulation and are midline defects that affect neural tissues, their coverings anywhere along the neuraxis, or both. They are heterogeneous malformations, and the terms used to define them here (see Box 8-1) are based on clinical descriptions and the presumed embryological defect (Lindseth, 1996; Volpe, 1995). The terminology in the literature may vary.
NTDs are not to be confused with spina bifida occulta, a common radiographic finding that does not involve neural elements, or encephalocele, a protrusion of meninges and brain tissue outside the cranium, most frequently in the occipital region.
BOX 8-1 Forms of Neural Tube Defects