Causes
The exact cause of spina bifida is still unknown. Something goes wrong in the first two months after conception but experts don't really know why. It's been shown again and again that women who have low levels of folic acid (a B vitamin) are more likely to give birth to children with neural tube defects. The assumption is that when the fetus's genes give the order to construct the spine, the necessary chemical building blocks are missing.
There's almost certainly no single spina bifida gene, and it can occur in any pregnancy, yet some people are predisposed to it. Women with a family history of neural tube defects are at increased risk of giving birth to a baby with such a defect. Women who have already had one pregnancy that resulted in spina bifida, whether or not they carried it to term, run about a 2% risk of neural tube defects in each subsequent pregnancy. This is more than ten times the average risk. Still, more than 90% of cases of spina bifida occur in families with no history of the condition.