Improper closure of the embyonic neural tube causes neural tube defects. This closure happens sometime between 6-8 gestational weeks (very early in pregnancy). The location of the abnormal closure along the neural tube determines where the defect results (cervical spine, thoracic spine, or lumbosacral spine).
Why the neural tube closes abnormally is unknown. It is thought to have a multifactorial basis. Also the different types of neural tube defects (anencephaly, spina bifida, and encephalocele) may have different etiologies.
Dietary issues in the mother, immunologic abnormalities involving folate receptors, genetic abnormalities in how the neural tube closes, maternal hyperthermia, use of certain anti-epileptic medications (valproic acid) at the time of conception, maternal obesity, and some chronic maternal diseases (diabetes mellitus) have all been shown to increase the chance of having a baby with neural tube defects.
Research has shown that low maternal folate or vitamin B12 levels or abnormal handling of a molecule called homocysteine (impaired re-methylation) can contribute to a defective closure (and increase the risk of having a baby with a neural tube defect). Folate deficiency has been most stronly implicated in increasing the risk of neural tube defects. Dietary issues that could cause these abnormal maternal blood levels (low folate or vitamin B12) include decreased consumption of meat and fresh vegetables. To decrease the chance of women having infants with neural tube defects many countries supplement the food supply with folic acid. In the United States all enriched cereal and grain products have supplemental folic acid. However in addition to this all women considering pregnancy should take a folic acid supplement daily to decrease the chance of having an affected baby (at least 400 mcg).