Normally the closure of the neural tube occurs around the thirtieth day after fertilization. However, if something interferes and the tube fails to close properly, a neural tube defect will occur. Among the most common tube defects are anencephaly, encephalocele, and spina bifida.
Pregnant women taking medication for epilepsy have a higher chance of having a child with a neural tube defect. Also, some research has shown that women with folic acid deficiences also have a higher chance of having a child with a neural tube defect.