 Closed NTDs occur when the spinal defect is covered by skin.Common examples of closed NTDs are lipomyelomeningocele, lipomeningocele and tethered cord.Anencephaly (without brain) is a neural tube defect that occurs when the head end of the neural tube fails to close, usually during the 23rd and 26th days of pregnancy, resulting in an absence of a major portion of the brain and skull.