A neural tube defect (NTD) is a birth defect that occurs when the spine, the brain, or the bone and skin that protect them do not develop properly. The most common type of neural tube defect is spina bifida, in which the spinal cord or spinal nerves may bulge out through an opening in the bones of the spine.

The neural tube is the part of a developing fetus that grows into the spinal cord and brain. Normally, the bones of the skull and spine grow around the brain and spinal cord, and then skin covers the bones, creating the neural tube. A neural tube defect occurs when this process doesn't happen normally.

Neural tube defects can be detected with prenatal tests, such as ultrasound and amniocentesis. In spina bifida, treatment depends on the severity. Surgery may be done to repair the spinal defect or to correct complications. Physical therapy, braces, and other treatments may be necessary to help the child with problems resulting from nerve damage.

Anencephaly is the second most common type of neural tube defect. In anencephaly, the infant is born with only a partially formed brain and spinal cord. This condition is always fatal.

Neural tube defects may be prevented if a woman takes folic acid before becoming pregnant and during the first 6 weeks of pregnancy. But often a woman does not know she is pregnant until after the first 6 weeks of pregnancy.